what are those suppose to do?

What is the purpose of those buttons?

MC = Memory Clear
M+ = Memory Add
M- = Memory Subtract
MR = Memory Recall
they're good for doing multiple calculations, add them to the memory when you have a subtotal, recall them when you want the grand total. I use it all the time at work.

These are useful if you can't either figure out parentheses, or can't imagine appropriate appropriate parentheses in your mind.