erics wrote:The nut forces the mushroom end of the tube against the ID of those male threads. It is a very common arrangement.
Similar looking but less expensive fittings from Home Depot etc are not made to accept compression bulbs in the male end. Almost all of the cheap ones are NPT anyway - they can be forced into the mating threads and made to seal with enough tape and force, but they're not the right parts to use.
Only high quality, purpose made pieces have the smooth, concave inner mating surfaces that seal against a compression bulb. The ones to which Alan and you provide links are both made like this. But the US standard threaded elbow adapters found at our big box stores do not have suitable mating surfaces - the bulb is pressed against the unfinished ID of the male end and seals only against a rough edge.
Using them is false economy - I just wanted to help the OP avoid this mistake.