If you can't clearly hear yourself playing when you're onstage, how can you know that your sound is where it should be? This is why stage monitors are so important. They give you a way to listen to your own performance while it's happening, so if you need to make an adjustment to your equipment, you'll know. On top of that, stage monitors will also help you and your bandmates stay synchronized since you'll hear the music with precision instead of having to work from muddled sound bouncing around the venue.
There are two primary kinds of stage monitor: powered and unpowered. The difference between the two is simple enough. Powered monitors have their own power amp built in, which means you don't have to connect them to a standalone one. If your main PA speakers are connected to a power amp and you've got spare outputs, then an unpowered (or passive) monitor makes sense, since you can just add it in. On the other hand, if you want a monitor that can work independently of other hardware, a powered model is a great option. Aside from the choice between powered and passive, you'll also find that there are some specialized monitors here that make a lot of sense for specific uses. As an example, the TC Helicon VoiceSolo FX150 is designed especially for vocalists, and can even be attached to a mic stand for convenient positioning.
There are also plenty of monitors built primarily for use in clubs, while others are intended for bigger venues. These are a few of the things to keep in mind when picking out stage monitors: for the best results, match your new monitor to your needs. There's no doubt that stage monitors are an important part of a live sound setup - it's a simple fact that listening to yourself is vital to playing at your best. These cabinets are also naturals for your rehearsals and practices. It's no accident that monitors are virtually always seen onstage with live musicians - it'd be hard to nail a gig without them!