I think Lucas was simply looking for a nuclear analogy or something comparable to the carpet bombing the U S did in Vietnam and Cambodia. It needed to look like it made sense.

That said, the Death Star is easy to retroactively justify. There are plenty of planets / dwarf planets / large rocks that have tasty minerals in their core, and are not otherwise usable for settlement or even an outpost. A super-laser would be a great tool at lower power settings to crack such rocks open to save on drilling, much the way hydraulic fracturing is used today. If the Imperial government is more inclined towards military spending than civilian projects, it then makes sense to sell it as a Mother Of All Lasers wrap it in a mobile space base.

That's how it goes in my headcanon, but like the IMPS videos, the troops are allowed to put sexy nose art on their mechanized armor.