Apart from the great answers talking about phrasing and emphasis, another purely technical consideration is synchronization.
If you don't have bars, it's very difficult to get everyone in an ensemble to rehearse a specific passage or phrase together. You could add rehearsal letters to a free-time score, sure, but it's very easy to say "ok, let's try it again from bar 58."
You also need some kind of sync points with the more complex and faster meters.
If you can, take a listen to the fast passages of The Sorcerer's Apprentice (Dukas) for instance. You might imagine this is in 9/8 and beaten in three - but it's not. It's in a very fast 3/8 and beaten in one. Without meter and bars, you could never get everyone together - it would be chaos.
(The joke here is that he didn't use 9/8 because Disney was paying him by the bar ;-)
There are also cases where composers like to play with the emphasis. Alfred Reed, for example, likes to use 5/4 bars but alternate the emphasis: 2+3, 3+2, 2+3, 3+2. Again, bars here help to keep you aligned as you play. Some composers (no name occurs to me at the moment) also score in one time signature, but place accents so that the music sounds like it's in another (these really mess me up).
Hope that helped too :-)
Edit: Here are a few interesting usages of meter from "El Camino Real" (Reed).
- Showing the emphasis split within a bar - the conductor will usually conduct here in three, with the downbeats on the emphasis points.
- Lazy meter changes - the double time signature here tells us that Reed's going alternate 4/4 with 3/4, and isn't going to write the changes in every time. This only lasts for 4 bars though.
- Meter change with emphasis - this section, which is a rather slow fandango, Reed starts out in 5/8 (emphasis 3+2), but as the fandango section gets going, goes into 8/8 beaten in 3: 3+2+3 per bar. Notice that in these long bars there are no accents written. The emphasis guide tells you how to stress the first note of each group.