Hi!
I sing lots of music and I also accompany myself on the piano. Other than to confuse the performer, what is the purpose of time signature changes in the music. Right in the middle of a phrase, a song might change from 4/4 to 2/4 and back to 4/4. Why??

Thank You.

I think this usually has to do with the rhythmic feel of the music.

The time signature doesn't just indicate note timing, it also tells you something about the pulse of the music and how pulses are grouped.

4/4 tells you there are 4 pulses in each measure and the first is usually emphasized while the following three are weaker (sometimes the third gets a secondary emphasis, but not typically as strong as the first). So you have something like:

DAH-duh-duh-duh | DAH-duh-duh-duh | ....


2/4 (or 2/2), on the other hand, usually implies something more like:

DAH-duh | DAH-duh | ...

(There are variations to all this, but that's the general idea).