I have already removed the mag safety from my fs .40. but I would like to know why you need to reinstall the sear disconnect. What purpose does it serve?
doesn't seem to do anything.

There are several threads on this topic at this forum with exactly the information you seek . . .

__________________
Wisdom comes thru fear . . .

The Sear Disconnect is a safety device that does not impair the function of the pistol in any way (like the Mag Disconnect), so there's really no reason TO remove it, other than bragging rights to say that you did.

If you NEVER use the Sear Disconnect when field stripping the pistol and plan on keeping your M&P forever and ever and ever, then hey... It's your pistol... Do what you want with it.
BUT...
If you're someone that flips their handguns and never keep one for more than a Year or two, keep it in there, so you don't forget to put it back to stock before trading/selling.
It's NOT in the way, it doesn't hinder anything, so why remove it??