The main benefit is that a 10-20mm lens is an ultra-wide zoom, while 18mm on your 18-250mm is only moderately wide on an APS-C camera like the 550D. The jump from 18mm to 10mm is very large in angle of view, so it’s not a small difference.

A dedicated 10-20mm can also be better optimized for wide-angle performance than a superzoom. Superzooms trade convenience for compromises, and the community answers note that the 10-20 shows lower distortion even at very short focal lengths than the 18-250 does at 18mm.

So the reasons to buy the 10-20 are:

• much wider field of view
• access to focal lengths your 18-250 simply doesn’t cover
• typically better wide-angle image quality and distortion control

If you rarely need anything wider than 18mm, the superzoom may be enough. But if you want true ultra-wide views for landscapes, interiors, or dramatic perspective, the 10-20mm is a meaningfully different lens, not just a slightly wider version of what you already have.