In order to get round stars, you need to keep a given star on the same exact tiny pixels over the length of your exposure. In order to do that, you need to de-spin the sky above you (polar alignment, tracking rate, mount mechanical errors), and offset external forces (wind, ground vibration, etc.), and correct for imaging train errors (focuser droop, etc.).
Basically, doing a good PA and engaging tracking is a necessary-but-not-sufficient. You do need do do those things, but there are other factors that will still mess you up if that's all you do.
If you go unguided, you can still image - but you're at the mercy of a bunch of uncontrolled factors and you will need to stick to short exposures and be prepared to discard some fraction of your shots. If you guide, it helps correct some of those multi-factor tracking errors, so your potential exposure length grows, and you should have far fewer discarded images.
Much more reliable....
Mounts: ES PMC-8 EXOS2
Scopes: SV 102EDT, ZWO 60/280 Guide
Cameras: Nikon D5300, Altair GPCAM2 290M
Software: ASCOM, CdC, AstroTortilla, BYN Pro, Sharpcap, PHD2
Computer: Thinkpad x230, Win7Pro/64