I heard they sometimes need to use powerful computers or see the experiment and results.
Theory is instantly put into trash when experiments disprove it. Experimental verification has the final say in deciding the fate of a theory. The problem is experiments aren't always possible. The energy requirements could be too high or time taken for computation could be greater than the age of the universe. It could be anything. Hence approximations are sought after. This at least gives an idea that we could be on a right track when making theories. Developing two theories and realising at the end that they're fundamentally contradictory.. Now that's a nightmare. We don't want that to happen. We want our theories to agree with each other. So computer stimulations and approximation in the lab itself is a key part for success for any theoretical physicist. Super computers are needed too for high end computations.
Theorists definitely work with experimentalists and use computational tools. However, the vast majority do not physically work in lab at all.
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