It’s a short rest period for the musicians and conductor–once you see how much activity goes into a performance, you’ll understand why they need a break! Listening to music is also an intense activity (even if considerably less physical), and a break in the middle helps the audience concentrate better in the second half. Some concerts, though, have no intermission because it would interrupt the flow of a long work. Check the program before the concert so you know what’s coming.
Most intermissions are fifteen to twenty minutes long, which gives you time to socialize with your companions, get a drink or a snack in the lobby, visit the facilities, or simply sit in your seat and read the program notes. Do whatever puts you in a good frame of mind to hear the second half of the concert.