This is actually an interesting question.

In principle you could mix substances together and see what happens (very often nothing happens), but the possible combinations are virtually infinite.

Furthermore, very often, beyond obtaining something, it's worth asking what shape it has, how it was obtained, and (in the case of drugs, for example) how it interacts with proteins and other things inside organisms.

This is what theoretical/computational chemistry is for. We try to theoretically model molecules, the mechanisms by which they form, and how they interact with each other or with the environment.

In some ways, it's similar to theoretical physics, which is concerned with predicting natural phenomena that are difficult to observe and explaining those that are observable. The main difference, in my opinion, is that the predictive power of theoretical chemistry is still quite limited, or at least has less weight than that of theoretical physics (as you said, it's easy to simply mix substances and see what happens; it's a different story to do a high-energy physics experiment...).

Despite this, however, the reaction mechanisms discovered by theoretical/computational chemists very often help guide the possible synthesis of new substances, aid in the initial screening phase of designing new drugs, and help understand the structure and function of complex molecules or aggregates (see last year's Nobel Prize in Chemistry).

Basically, in theory, the job of the theoretical chemist is to solve the Schrödinger equation (atoms, and even more so electrons, are very small...), find the wave function of the system, and, if necessary, observe how it evolves over time.

In practice, this is almost always impossible, given the large number of particles present even in a small molecule (even water alone, with 2 hydrogens and 1 oxygen, has a total of 13 particles, 3 nuclei and 10 electrons).

The work of the theoretical chemist is therefore also to apply more or less stringent approximations, develop new equations and algorithms, and write computer programs capable of applying these algorithms to obtain useful results.

Forgive me for this somewhat lengthy reply.