Well, in a way, you don't need bars. Not for the sort of thing you were describing. If you just set a metronome up and start rapping away, you probably won't have any problems. But if you want to rap with a band, or anyone else, or if you want to write your music down, bars are necessary.
There are several reasons for the concept of bars:
- Communicating metrical information. With your 60 BPM example, changing the meter actually does affect a lot about the song. Think about a waltz vs. a march. In a waltz, which is in 3, you count
1 2 3 |1 2 3...
With an assumed metrical accent on the first beat of each measure. A stead pulse from a metronome does not carry this information.
What if you had it in groups of four? Then, in most western circles, you would get:
1 2 3 4 |1 2 3 4...
In these examples the BPM did not change, however the feel of the song would dramatically change.
Once again, this is not communicated by a steady pulse from a metronome. Now, if you were rapping or singing or playing by yourself, you would almost undoubtedly use one of these, or another simple/compound meter, though you may not realize it. This is fine. But if you want to play with a group, especially if they're sight-reading something that you wrote, having the concept of bars coupled with time signatures is necessary.
Sight reading. You might be wondering why we can't just write out which beats are stronger without measures, and technically, we can. It's just really hard to read. Bars break the information down. It's much easier to feel measures than it is to theoretically know that certain beats are stronger than others. In a way, when playing as a group, nobody's really thinking that hard about
measure, measure, measure, but counting becomes a lot easier when the numbers stay smaller.
Rehearsal. In orchestral settings, rehearsing one long stream of music would be quite challenging. First of all, if you got lost, it would really suck. Second of all, there's a lot of stop and go in rehearsals, and having measures as reference points, which give each beat their own unique feel in relation to the others (more having to do with meters) is profoundly useful. Thirdly, conducting is a lot more helpful when the conductor can show each beat in a measure by waving his hands in a different direction. For our 4 beats example, the conductor might show
DOWN right left up
With measureless music (which, actually, does exist) this isn't an option.
At the end of the day, measures are descriptive of the way we hear music and rhythm. They are also helpful for studying music as a group or even an individual because they allow places of reference.