Why was the base_fee for the routing fee calculation of the Lightning Network included?

Currently and as defined by BOLT 07 on the Lightning Network the fees for forwarding an HTLC of amount amount_to_forward are computed as:

fee_base_msat +  amount_to_forward * fee_proportional_millionths / 1000000 

I am wondering why the base_fee_msat was actually included and which concrete problem does it solve that could not be solved if the base_fee_msat was dropped?