Monitoring allows you to get real-time feedback during a recording session.
But it also allows you to hear your recorded tracks when you play them back during mixing.
I recommend using headphones during the tracking, or recording, part of your projects.
This is due to the fact that the output of the monitor speakers can bleed over back into the mic while you're recording.
In extreme cases, this leads to a whole different type of Feedback that you'll want to avoid at all costs.
You may want to put the headphones on only 1 ear to allow you to hear your un-amplified and miked vocals with the other ear.
Sometimes this can help you give a more natural performance... it's tough to just act normal when you are hearing your voice in a very unnatural sort of way.
If you decide to do this make sure to cover the other side of the headphones to prevent the same bleed-over problem mentioned above.
This is particularly necessary when using a metronome to control your tempo because the clicking of that track will definitely show up in all the wrong places and could ruin a good take.
Then later when mixing your tracks together, studio monitors allow you to hear your project over the air, which is a more representative way that it will eventually be heard.
Sometimes the over-ear headphones sound a little "too good to be true" due to the fact that they sit right on your ears.
Basically both methods have their place in your studio, but if you are on a really tight budget I recommend the headphones first, then the monitors when you can afford them later.
One very useful tool when setting up your system to monitor your recording sessions is to assign custom sub-mixes to each musician in your group.
This provides each musician with total control over how they hear the rest of the arrangement, and over their own relative levels and panning.
If you are a solo act then this capability won't make any difference, but if you have even 1 other person it can make a big difference during both rehearsal and Tracking.
Chances are you both will want slightly different mixes, and with this method you don't have to compromise on what that means, allowing you both to give your best performance.