Why do we use conversion factors instead of just multiplying or dividing as needed? what’s the advantage?

If you’re referring to stoichiometry, which usually involves the long t-tables of multiple conversions, then you’re not alone in wondering the purpose of using them. The purpose of conversion tables is to help keep track of what you’re doing. When quickly converting one value to another, it’s very easy to forget a particular piece to the puzzle, or to use a factor that doesn’t work for that instance. Teachers usually draw out a t-table every time they work a problem so as not to confuse students, but anytime you’re working with funky units or converting across systems (metric to imperial and vice versa) it’s a good idea to use them.