It depends on the speakers and your computer. Since 2012, the iMac doesn't have an audio in headphone jack or any other port that allows Line-In recording other than USB or one of Apple's specific connection types. So, I got an Alesis iO2, a two channel audio interface that connects through UBS and let's you plug speakers into the back. I have Alesis Monitor One unpowered studio monitors, and those are connected to an Alesis RA150 power amp, which plugs into the back of the Alesis iO2. This would be impossible without the audio interface....
So I would say yes.
Wow. You have in your hands some of the most advanced and powerful music production software on the planet. Surely you would want an audio interface to make it sing rather than limp along. How are you going to control momitor level, headphone level, etc. If you are going to connect a mic, it is essential (unles you are going to opt for a USB mic. There are very few pros who would choose that option). Any audio interface will sound better than the sound card inside the mac, but if you go for something like Native Instruments Komlete 6, it's affordable, user friendly and sounds amazing. There are other good ones, like Focusrite and MOTU, but M audio and the lower range of Presonus are not in the same league and the more advanced users would avoid them.
Hope this helps.