Hammer can be an efficient way to break high firewalls on important devices; it hacks for 1 PWR per firewall broken. However, because of its high PWR cost and cooldown, Hammer is not suitable for much else, as it can usually only be used a few times per mission at most.
Compared to Dagger 2.0, Hammer breaks the same number of firewalls, but has a higher PWR cost and a shorter cooldown. In practice, Hammer's high PWR cost typically prevents it from being used more frequently than Dagger 2.0 anyway. So there are usually better options than Hammer, but if you don't find them then it may be worth installing Hammer anyway. When combined with Charge, Hammer becomes equivalent to Dagger 2.0.
Hammer can be used more frequently when playing with Seed, so long as it is the first program you use that turn, as it gets the maximum benefit from Seed's PWR cost reduction. Hammer is also more viable in Endless mode, when many devices have high firewalls, and you have enough PWR generation from augments and items to use Hammer more frequently.