Let me answer the first question you have. Customization. THAT’s why you should build shelves as opposed to buying them from Ikea, Home Depot or any other store.
By building shelves yourself you can make sure to use every inch of space that you have and even build your shelves around the boxes you know you’re going to be storing on them. No wasted space, no fits when you find out your massive stock pot is 1/2 ” taller than your biggest shelf.