/\/\/\ What Moresound said.
You CAN mix on cans or computer speakers or home stereo systems but they're gonna be hyped in certain frequencies and just not accurate. With enough burned CD's and time, you can find the sweet spots in your mixes. But it'll come down to.........if it sounds good on your system, then it's wrong. Once ya learn the tweaks that'll make it sound right on other systems, it's not gonna sound right on yours.
See what I mean?
Monitors (can) give you more accuracy in detail which will help in how your mixes translate to different systems.
Like if you did all your mixes on the 5.0 system, they'll sound good on that 5.1 system and others like it. Not so much on car stereos or home systems or regular computer setups with 2 speakers and a sub.
It's about how they translate.