These calculators cover the numbers that come up when you are judging an investment rather than planning a savings balance: what a holding actually returned once you account for time, what a company pays out and whether it can afford to, what you are paying for each pound of earnings, and how a bond behaves when rates move.
None of them tell you what to buy. They convert the figures in a set of accounts or a broker statement into something comparable across holdings, which is the part that is easy to get wrong by hand — particularly annualising a return, where the intuitive answer (total gain divided by years) is the wrong one.