The main purpose of the tunnels is to allow CM's to get to each land so noone is ever out of place to guests based on their costume. There are HR areas, payroll, training rooms, break rooms, stock rooms and more. This area also helps get purchases to the front of the park or to the section to sort for resort delivery if you opted for that.
The other purpose of the utilidor is for trash. The AVAC system runs through the utilidor making it easier to process trash, out of guest view (and smell).
As others have stated wardrobe/cast lockers are currently there (they are being moved to a new building out in the cast member parking lot). The cafeteria (or "Mousketeria" as it is called), offices, some breakrooms, HR, schedueling, maintenance, the character zoo (where the characters live ) etc. There are lots of pargos going back and forth down there as it is easier to go from one end of the park to the other (the only other vehicles that are allowed in the tunnels are the Brinks (money) trucks and ambulances) It is really not that glamourus (sp?) but it is of interest if you have not seen it. The Keys to the Kingdom tour goes into the "tunnel" and I believe tha Bacstage Magic tour does as well, if you want to get a chance to see it.
As for your question about access, some (not all) entrances have elevator access, most are just stairs.
and we did get a brief look at the Utilidors area. Not glamorous in the least, but inrteresting to see how some things work. I would never do it if you were worried bout losing any of the magic though because it could "ruin" it for a real Disney nut -- or a kid -- though I believe kids under a certain age aren't allowed to do keys to the kingdom for that very reason.
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