Dataframe columns having individual indices

I have noticed an interesting behaviour which I haven't seen in the documentation:

Each column inside a dataframe can have its individual index!

df = pd.DataFrame(np.arange(12).reshape(4, 3, order='F'),
                  columns=list('abc'))

df
   a  b   c
0  0  4   8
1  1  5   9
2  2  6  10
3  3  7  11

Assign index to column b:

df['b'].index = [-1, 2, 4, 5]

Different indices for different columns, but they all share the same dataframe index:

df['a']
0    0
1    1
2    2
3    3
Name: a, dtype: int64

df['b']
-1    4
 2    5
 4    6
 5    7
Name: b, dtype: int64


df.loc[:2, ['b']]
   b
0  4
1  5
2  6

df.loc[:2, 'b']
-1    4
 2    5
Name: b, dtype: int64

Is this described somewhere in the documentation?

Why can this be done in the first place? And can this be useful for something?

from Dataframe columns having individual indices