Against 1.Nf3 TDKS is actually a very logical continuation. White, like Black, does not directly occupy the centre with pawns; White prevents e7-e5 with his knight move, Black restores this possibility.
The whole thing acquires its own significance if White does not continue with 2.d4 (transition to TDKS against 1.d4) or 2.e4 (transition to TDKS against 1.e4). The former is predominantly the case, the latter very rarely (Nf3 players do not tend to transition to possible open openings). 2.c4 transfers into TDKS against 1.c4.
This leaves 2. g3, the most played move after 2.d4 in the board position. Black answers 2… e5 – and for the Reti player (Reti opening 1.Nf3) it is already unusual to uncomfortable to have to deal with possible attacks (e5-e4) against his knight.