The SD card is for extra storage of 'some' items. Although you can't move most apps there (phone running any OS lower than "Marshmallow" can't run most apps from card) you can move some of them. You can also move Photos, videos, music, etc. Those items also take up a lot of space on a phone, so it is good to move them to a card.
A side note: I would also suggest backing up any personal data from the phone and/or SD card to a computer or cloud account, so you don't lose anything should the phone be lost, stolen, break, etc.