It is simple, in a super market, finding out things are easier as they are properly arranged, also it serves as a outing. Going to a normal market is not as interesting as going to a super market it ..hence people prefer going to super market. Plus you get some rebate on the net amount ..proves cheaper as compared to normal market.
Supermarkets give us scope to choose any product according to our choice amongst many brands. They give garanteed products at reasonable price. Normal markets does not give such options freely. Moreover it is better to change standard of living with the passage of time.
All things are avilable in a super market. Also different brands are available allowing to choose the brands that we want and it can be purchased at a cheaper rate. All the needed things can be brought in a single place.
Super markets provide your requirements under a single roof. For sarees you go to a saree center. For under garments to another shop and you will be roaming from shop to shop to get all your necessities. It's a question of convenience.
The supermarkets have a huge range of products in each category that helps you to choose and compare prices, ingredients, quantities etc. so that you can choose a product that suits your needs perfectly well. There is also a provision to buy stuff like vegetables and fruits etc. in a single piece or small quantities which is normally not available with local grocers and small shops. This in my opinion is the best thing about supermarkets. Also, you can be sure of the weight which is a problem with local markets and their weights also are quite doubtful in many cases.
I think it is more because of the convenience, we are able to get everything under one roof.However, I still go to our local market for vegetables and fruits since we get local varities and fresher products, the same with regard to fish and sea food...