What a Loudspeaker Needs to Create Sound
A loudspeaker needs two types of signals in order to work.
First, speakers need to access the music or movie soundtrack. This is provided in the form of electrical impulses (the audio signal).
Second, in order for the speaker to take the electrical sound impulses and convert those impulses into actual sound that you can hear, the speaker needs to be physically connected to an amplifier, which can be powered either by a battery (most applicable for portable devices) or AC power.
In a traditional wired speaker setup, both the soundtrack impulses and the power needed to make the loudspeaker work are passed through speaker wire connections from an amplifier.
Wireless Speaker Requirements
In a wireless speaker setup, a transmitter has to be physically connected to preamp outputs on a receiver, or, in the case where you have a packaged home theater system that incorporates a built-in or plug-in wireless transmitter. This transmitter then sends the music/movie soundtrack information to a speaker or secondary amplifier that has a built-in wireless receiver. However, since power cannot be transmitted wirelessly, in order to produce the audio signal that is wirelessly transmitted so you can actually hear it, the speaker needs additional power.
This means that the speaker still has to be physically attached to a power source and an amplifier. The amplifier may be built right into the speaker housing or, in some cases, the speakers are physically attached with speaker wire to an external amplifier that is powered by batteries or plugged into the house AC power source. Obviously, the battery option severely limits the ability of a wireless speaker to output adequate power over a long period of time.
When Wireless is Not Really Wireless
One way that so-called wireless speakers are applied in some Home Theater-in-a-Box Systems that tout "wirelesss surround speakers", is they simply have a separate amplifier module for the surround speakers.
In other words, the main receiver unit has a built-in amplifier that physically connects to the left, center, and right front speakers, but has a transmitter that sends the surround sound signals to another amplifier module that is placed in the back of the room. The surround speakers are then connected by wire to the second amplifier module in the back of the room. In other words, you have not eliminated any wires, you have just relocated where they go. Of course, the second amplifier still needs to be connected to an AC power outlet, so you have actually added that.
So, in a wireless speaker setup, you may have eliminated the long wires that typically go from the signal source, such as a stereo or home theater receiver, but you still need to connect the "wireless" speaker to its own power source in order for it to actually produce sound. This can also limit speaker placement as distance from an available AC power outlet then becomes a major concern. You may still need a rather long AC power cord if a convenient AC outlet is not nearby.
A more practical application of wireless speaker technology that is gaining a lot of popularity, is in a growing number of powered subwoofers. The reason that wireless subwoofers make a lot of sense is that they are typically self powered already and, thus, have both a built-in amplifier and the required connection to AC power. Adding a wireless receiver to a subwoofer does not require a major redesign cost.
Subwoofers are sometimes located far from the receiver they need to receive the audio signal from, incorporating a wireless transmitter for the subwoofer either built-in or added to a Home Theater Receiver or Preamp and a wireless receiver into the subwoofer is a very practical idea. The Receiver transmits the low-frequency impulses to the wireless subwoofer, and then the subwoofer's built-in amplifier produces the power required to allow you to hear the sound.
This is becoming very popular on sound bar systems, where there are only two components: the main sound bar and a separate subwoofer. Although the wireless subwoofer arrangement eliminates the long cable usually needed, and allows more flexible room placement of the subwoofer, both the sound bar and subwoofer still need to be plugged into a AC wall outlet or power strip. However, it is a lot more convenient to find a power outlet for one speaker (the powered subwoofer), than two, five, or seven speakers that make up a typical home theater system setup.
As you can see, when considering wireless speakers, there are several things to note. The fact that "wireless" doesn't always really mean wireless is certainly one issue, but, depending on your room layout and the location of your AC power outlets, some sort of wireless speaker option may be perfectly viable and desirable for your setup. Just keep in mind what speakers require to produce sound when you shop for wireless speaker options.
For more on wireless speakers and wireless home theater connectivity, read my article: What is Wireless Home Theater?.