Actually, even in pure mathematics, it very often is possible to do experiments of a sort.
It's very common to come up with a hypothesis that seems plausible but you're not sure if it's true or not. If it's true, proving that is probably quite a lot of work; if it's false, proving that could be quite a lot of work, too. But, if it's true, trying to prove that it's false is a huge amount of work! Before you invest a lot of effort into trying to prove the wrong direction, it's good to gain some intuition about the situation and whether the statement seems more likely to be true or to be false. Computers can be very useful for this kind of thing: you can generate lots of examples and see if they satisfy your hypothesis. If they do, you might try to prove your hypothesis is true; if they don't, you might try to refine your hypothesis by adding more conditions to it.
See also Oswald Veblen's answerOswald Veblen's answer which talks about doing similar "experiments" by hand.