Having more Weapon and Tome slots is almost necessary if you're planning to go deep on your run in Megabonk.
Every character begins with a fixed number of slots when starting a run. This is universal, irrespective of which character you have picked. If you're on a smaller map (tier one), getting more slots isn't that useful. However, the importance rises significantly the moment you spawn on a higher-tier map (tier two, for example).
