Benefits depend a bit on the case. In general MBB (vs competitors) won't budge too much on their policy as a negotiating tactic with clients.
Generally, expect "normal" five star hotels - e.g. Sheraton, Westin. However, this varies massively. I know of a case in a small town in England where they had to stay in what was basically a B&B (only hotel around). Also know of cases in the middle east where they stay at the Ritz/Four seasons, or at the W in Barcelona. Keep in mind that even if you're staying in a glamorous five star hotel, it probably won't matter much as you won't be able to use much of the amenities. Biggest differentiating factor is service - very accommodating front desk staff, helpful concierge, etc.
Other of these types of perks/benefits are broad but include
- getting status with airlines & hotel chains and collecting airmiles/hotel points (which you can then use to finance very nice holidays)
- Credit card points (depending on choice of credit card) through high spend (on a normal travel case you can spend anywhere from 1k-3k a week depending on cost of flights and hotels)
- Expensing nice meals every week (not just on travel cases for dinner, but also team lunch in the office, etc.). You also save a significant amount of money on food this way
- In some cases, doing laundry at the hotel (usually cases that require more than a week at client site)
Over time, I have stopped thinking of these as perks of the job that are desirable. I think of them more as ways to partly compensate me for having to be away from home 3-4 days a week.