It's for the A.I.R., or Air Injection Reaction system. Those pipes are just so the air pump can push fresh air into the catalytic converters, thereby completing catalyzation. Put more simply: lets the exhaust "burn completely".
If yours doesn't have any holes in it, leave it alone. Modifying this system will do nothing for giving you more power, plus unless it's done to entirety will lead to more problems.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch