The upside of running your own node is that you've verified every block yourself, all the way to the first (genesis) block. By running a node you're also part of the decentralisation of the network. Oh yeah, one more big upside is that you're able to increase your privacy if you set everything up right.
The biggest downside (in my opinion) of running a pruned node is that you should not import other keys/addresses. If your node is forced to rescan the blockchain, it has to download every block again, since a pruned node removed most blocks after parsing.
Decentralization is subjective, since you're only having a small portion of the blocks, you are not contributing that much to the network and just marginally. People won't exactly be able to rely on you for IBD for example, though they can still get whatever you've got on the disk. Privacy wise, it would require quite a bit more than running a pruned node and using it. Further setup is required.
You can import keys and addresses without triggering rescan. Afterwards, you can just rescan a portion of the blockchain and it won't require a reindex.
2. Is it necessary to run the full node?
3. What benefits will I get if I run full node?
2) No. Most people can do with a SPV node, if they're okay with sacrificing a bit of security and privacy. In fact, SPV clients are safe and secure for everyday use and if you can't run a full node, you can consider running an SPV wallet like Electrum.
3) You can validate every block and have a bit more privacy than SPV clients. It's a tradeoff but you would have to spend more time validating and more storage as well.
Actually it's always better to run a full node if you can, since you are not going to be depending on a third party to verify a block and as mocacinno said it improve decentralisation of the Bitcoin network.
Pruned nodes are also full nodes.
