Lots of people are running full nodes that are not used for mining, not using a bitcoin core wallet with it, and not using an SPV wallet linked directly to that node. In other words it is a network node (is there another name for this?) that validates transactions and validates blocks, and interacts with other network and non-network nodes.

Are these needed? How do they improve overall security of bitcoin?  I'm really asking here, not really sure one way or the other what effect they have.

If we had no network nodes and only mining nodes and nodes directly connected to wallets would bitcoin be less safe, more safe or the same? Like we might end up with just 100 nodes or something (1 for each large mining pool, a few for the large bitcoin service providers like coinbase, etc, and a bunch for people using their nodes with the bitcoin core wallet, and a handful of SPV users. Maybe 100 is a bit too small a number (what would the number be?).