For me its 3 things:

1. iTunes Match. This is the big one. I can't rip old CD's and charity shop compilations to Spotify for streaming. I use this every day.

2. Legacy Purchases. Alongside those old CD's I have nearly 20 years of iTunes purchases, many of which are not available to stream.

3. Artist recompense. Both pay a pittance but Apple pay twice the pittance. If Apple pays an average of 1p per stream I only have to listen to a 10-track album through 7x for them to make the same amount of money they would get from a 10% royalty of a £7 album sale. I easily listen to most albums a lot more than this.