Hi Solaso this is intended for future scaling,
If you can expand your drive 10 gigs might be enough today and expand it later, but the spec is there to allow for future growth of blocks as the size grows, updates to sentinel and DASHD as well the database of objects that will need storage space, 20Gig is like barley anything, the first year of the bitcoin blockchain wouldn't even fit on 10Gigs.
Think of it like this when you purchase you automobile did you tell them you only needed a 6 liter gas tank because you only drive 1 km to work and back....