Cos it’s in the original photo.
The M-16 came with an el cheapo clip on bipod. It was spring loaded, rather like a giant clothes peg and of dubious value, although I suppose,if you had the time and inclination, could improve the accuracy of shots made from the prone position. It was liable to be lost.
The original intention was as a replacement for the BAR/M-14 E1 that had existed in each fire team prior to the adoption of the M-16. In theory each fireteam still had someone designated as an automatic rifleman.
I’ve fired an M-16 on the range with one and I’d suggest that’s about all it’s good for. Pointless encumbrance otherwise, it can’t be folded up it has to be clipped or unclipped to the barrel. Potentially useful if you are spotting for a sniper I suppose.