Quoting the Pandas docs
pandas.DataFrame(data=None, index=None, columns=None, dtype=None, copy=False)Two-dimensional size-mutable, potentially heterogeneous tabular data structure with labeled axes (rows and columns). Arithmetic operations align on both row and column labels. Can be thought of as a dict-like container for Series objects. The primary pandas data structure.
So, the Series is the data structure for a single column of a DataFrame, not only conceptually, but literally, i.e. the data in a DataFrame is actually stored in memory as a collection of Series.
Analogously: We need both lists and matrices, because matrices are built with lists. Single row matricies, while equivalent to lists in functionality still cannot exist without the list(s) they’re composed of.
They both have extremely similar APIs, but you’ll find that DataFrame methods always cater to the possibility that you have more than one column. And, of course, you can always add another Series (or equivalent object) to a DataFrame, while adding a Series to another Series involves creating a DataFrame.