Several reasons....
The phone has an issue where the signal fluctuates and will lose it's connection. It will, at times, think it has no connection, even if it does, and when that happens calls and sms don't work. This is aggravated by the signal fluctuations. HTC is supposedly looking into it. Finally, the stock hboot doesn't allow GB roms.
By swapping these parts out, you reduce the fluctuations and how often it thinks the connection has dropped, boosted the signal strength, and allows you to use almost any rom.