It helps during the transition, and even afterwards.
France, by far a leader when it comes to nuclear (used to produce 90% of its gridpower this way) always used fossil fuel energy to complement it reactor fleet (each year fossil fuel produces 7 to 12% of France's electricity), mainly because maintaining enough reactors in order to cope with peak consumption (thus letting many underused) would be way too expensive, and also because nuclear load-following capacity is insufficient (bar over-provisioning reactors) due to their limited flexibility (at this game a gas turbine is way, way better): for security and burden-of-maintenance -related reasons one just cannot freely pump up/down the power delivered by a nuclear reactor, modulation limits and pauses are to be strictly respected.