Their curbside pickup in my town is free, no subscription required. If there's a problem with my order, they always fix it. Their employees are usually very nice.

They're always less busy than Market Basket. (I actually like MB quite a bit.)

The two other options within a ten minute drive of my house generally cost more than S&S. Since S&S is directly competing with MB in my area, they hold their prices a bit lower. It's also easy to shop mostly sale items from S&S. Most everything goes on sale at least once a month.