For convenience and security.

E.g. multiple wallets (paper) for long term offline storage, a mobile wallet for easy small payments, and a hardware wallet for secure large payments.

You know its been discussed in some threads but seriously still confused. Well this quote always posted here:

'Don't put all your eggs in one basket'

Hope you got it.

There are different options depending on your situation:

Core: if you have the bandwidth and storage capacity to run it. You also support the network by running a node.
Ligthweigth: You don't run the full blockchain, but a "central" chain is used to save bandwidth and storage on your pc. Good if you have a low capacity machine/internet or just don't use bitcoin too often.

Than there are also different types of lightweights, all with their different options and security impelmentations.