The real answer for why there were so many permanent airfields such as Binbrook 1940, Manby 1938, Coningsby 1940, Cranwell 1916, Digby 1918, Waddington 1916, Scampton 1916, Hemswell 1937, Kirton Lindsay 1940, Spittlegate 1916, Swinderby 1940, North Coates 1935 is the policy instituted by Trenchard.
He realised that aircraft would be in service for very short periods before they were superceded by newer better types. It was better therefore to create a proper infrastructure and only expand the aircraft inventory when required.
You mentioned hard runways; many hard runways were only built during the war and extended in the 50s for the newer types.
The hangars were a masterpiece of design. They have blast doors that would minimise fragmentation damage and soft blow-out roofs so that an explosion inside would be contained. Hangar services were also split with each half having separate plumbing and electrics.
If you look at the various officers' messes you will see that they are all of single story construction. If you go west and north outside the then presumed range of the enemy bombers operating from Germany you will find messes with 3 floors. These rear area airfields also have many more hangars for aircraft storage.
I will, if I have time check the other airfield of which I think there are 34. These were temporary airfields which were handed back to prior owners etc post war. One, Spilsby, was identified as a USAF cold-war recovery base for B47s. East Kirby nearby was an emergency airfield up into the 1960s. It was unmanned but had a newly resurfaced runway and fuelled goose-necks for a flare path as late as 1965.
And ultimately why so many? The many were, as said, required for the huge number of bombers required to take the war to Germany.