There are many reasons to build a fusion reactor: It keeps hot-neutron physicists, who you must recruit from among for weapons work, busy. It provides continu

There are many reasons to build a fusion reactor: It keeps hot-neutron physicists, who you must recruit from among for weapons work, busy. It provides continual practical challenges to plasma fluid dynamics physicists, who otherwise have great difficulty funding experiments. It provides cash flow to the (chiefly) military contractors who build the test apparatus. In this particular case, it lets you conduct tests for thermonuclear weapons concepts paid for out of a different budget. Any expectation of ever getting useful energy out would be the worst reason, because there will never be one solitary erg of that.