What's the benefit of destroying abandoned dwellings?

On some maps there are "Abandoned Dwellings" that you can destroy by dropping bombs of a certain grade, but I can't figure out what benefit there is in doing so. Is there a mana reward or anything, or is the benefit simply that you clear up space to build a tower?

Solution 1:

There are different benefits for destroying abandoned dwellings:

• Bonus mana gain, though this is usually less than the mana it costs to buy a gem of the required grade. If you have a spare gem, it could be helpful.
• Achievements, if you play normal mode you will be awarded with skill points for each achievements earned. The amount of skill points depends on the achievement. The achievement 'It was Abandoned Anyway' yields 1 skill point for destroying 1 dwelling and 'Ruined Ghost Town' yields also 1 skill point for destroying 5 dwellings in 1 level.

See here an overview of all the achievements and there rewards. The achievements for destroying dwellings start at 101.