Yeah... I started out using a 100% sidearm cast but soon learned that if the stream is too narrow, or, you are facing a narrow opening between two bushes (or the like) the side arm cast is likely to snag either side of the opening. By making the loop open up underneath the main fly line you can still get a fly in there....
Depending on vertical clearance you can land the fly gently in the opening, or, get it to bounce/skip on the surface a couple times as it skitters into the opening... Sometimes this extra commotion is necessary to draw the strike...
Never considered doing a side arm cast to get at that undercut beneath my feet, will have to try that this spring on my local water.
I once knew a drug addict... He told me I have a fishing problem!