The moment that we say "이것저것" is when we cannot or don't want to decide, ususally to buy some food at a foodcourt. There are so many food and you are so undecided to pick what you like to eat. You can say like "그냥 이것저것 주세요" (I ll just take anything you pick for me (to the person who serve your food)) In this case, unspokenly, you are telling your food server that you don't care too much about the food and also you trust their taste for you. Or sometimes you feel like wanting to avoid some direct question from others. Like when someone, who think you spend too much money on cloths, catch you coming home with a bunch of cloths, and they ask you what you bought this time, you could say "그냥 이것저것 샀어요" (I just bought something not too fancy) This time, you leave out a detail information so that you can draw their attention to somewhere else. Secretly, you are telling them to mind their own business by saying "이것저것" instead of naming each item that you bought from a shop.

신우열 is correct

Thank you ^^

a little of this a little of that