May I ask what a main limit switch is and what it's purpose is? I have a 150,000 BTU sparker igniting furnace model 58GS150-1A (Carrier) Series 101.

Main limit is a safety device.
Its purpose is to help the furnace from going into terminal meltdown.
In other words it is a very important part and if you have a bad one you need to find out what the problem is and then replace the high limit.
Dirty filters and A/C coils can cause this as well as other things.