Speaking from experience, it's great to be able to tuck a Merlin, Strumstick, or Cumberland under your arm, but it's even better (and easier) when you can let a strap take some of the weight!
Allow yourself more balance to move freely and play better!
It's a must-have accessory.
NOTE: This strap is specifically made by Seagull for their D and G model Seagull Merlin M4s, but would work well for any instrument with a single strap button.