Beside of the lookup_ext() function as a function call there are three more options. You can use the lookup() function in the mapping formula which is simple and easy enough to read. You can use lookup_ext() function as mapping but it is difficult to read. And there is the lookup_seq() function, which allows for equal conditions plus one additional greater in it. Why are there so many different ones?
The answer is quite simple: lookup() and lookup_seq() both exist for backward compatibility only. The lookup_ext() function is a superset which has all the functionalities and more.