IPsec facilitates encryption and authentication with ESP. With tunnel mode, IPSec is also providing tunnelling. More importantly, one can use IPsec in tunnel mode, along with transport mode (i.e. encapsulate one over other).
Hence, IPsec is enough for:
Whereas, Layer two Tunnelling Protocol (L2TP) is meant only for tunnelling. Then, for encryption, it uses IPsec.
My question is why IPsec has to support L2TP (kind of self-respect), where IPsec is capable of both encryption and tunnelling?