Eurorack modules are capable of much higher output levels than line level gear. You can send eurorack signals directly into line level gear if you are very careful, but a mistake could damage the line level gear. It's a lot safer to have an interface module. Also, although line levels going into eurorack gear can't damage anything, the level does need to be boosted or it will be very quiet. I have the Audio Interface II and am quite happy with it.