1 Example: Eastline Airlines fly CGN - STR - DME
Berlin has got 3 Airports: TXL, THF and SXF
SXF was the Airport of DDR in Berlin
THF was the Airport of BRD in Berlin a long time ago.
But (perhaps you know it) it´s not a big airport, and due to that TXL was built.
Today, TXL has no capacity anymore, as most flights ex Berlin were served from TXL and the Airport is to small for 3 Mio. inhabitants.
The most charter-flights and flights into east (Russia, Ukraine etc.) are served from SXF. Further SXF is the only Airport with no limits on flying at night.
THF is more central. It´s often used by people who make business trips.
Every Airport here has got his advantage! Also, I don´t know, why you´ve got a problem with much airports!
But good news for you:
There are plans to close THF in a few years.
SXF will be extended to "Airport Berlin Brandenburg" and when it´s ready, TXL will be closed too. "Berlin Brandenburg" will be the only Airport in Berlin.