Inserting machines make processing mailers much easier. Load a stack of envelopes and load the bins with whatever content you'd like to be mailed out and then just press go! Envelope inserters often fold the sheets too with a letter fold to make them fit in the envelope. Some of them will seal the envelopes with a liquid to close them for mailing.
Inserters can get large and expensive so it is important to know how many supply pockets you need for your application. Mailing runs can also be very long so equipment with a higher duty cycle will hold up over time and remain reliable when you have lots of work to do.