They prevent the subgrade soil from being disturbed due to precipitation (and then workers or others walking/driving over it and causing rutting, etc. The flat surface is good, as you would suspect, to ensure proper steel placement.  You could use a good 19mm crushed stone in lieu of blinding concrete (mud slab).  Some specify to put a 150 mm layer of sand below - I wouldn't as it will become wholly disturbed when you drop the mudmat concrete on it and with the workers walking about on the sand.

now that i understand the question after bigh clarified the terminology for me (i was picturing the question asking about waterproofing...not weather proofing): for what it's worth, a mud slab is much easier to deal with if wet weather is approaching (or even for subgrade susceptible to shallow ground water). crushed stone tends to get muddy when the silt washes over it so you end up loosing all the stone when you clean out. if the subgrade or excavation is good to begin with (after being excavated a couple of inches deeper below planned grade), then slosh a couple of inches (most specify 3" here but i think 2"+ works just fine...much less than 2" and it gets iffy pretty quick) of very wet concrete in the bottom. i've seen folks try to place a mudmat with 3" slump concrete and that is just silly. you end up with a very rouch surface that can't be cleaned off very easily and the field guys have a hard time getting the thing more or less level.

there's been times where we ended up having to pull the mudmat back out before pouring because the excavation sat under water for a month (rained every other day and we didn't have enough time to tie all the rebar and didn't want to end up having to pull it all back out to clean off mud mat). that particular instance is the only one i can think of where the mud mat didn't work quite as we expected...however, we pulled the mud mat out plus maybe an inch of the underlying soil and everything else was good-to-go so the mudmat still performed well and was worth the minor expense). even if you drop a few hundred dollars on a mudmat for a sizable excavation, it's very quickly made up in the time spent by the workers waiting around to clean up the area and or additional undercutting that might be required due to softening of the surficial soils.

they also stop the reinforcing chairs from sinking into the mud ensuring cover.

Cool thanks guys