Adjustable handlebar - do I need it, and why are there different ways to do it?
TL;DR: An adjustable handlebar is necessary for too tall, too short, or two different sizes of parents. There are two approaches to adjusting a handle - by telescoping it out (= more space for tall people, though, might be wobbly) or by rotating it (favorable for the center of gravity, might be weaker). Be careful of setting the handle to the lowest comfortable position to get the best driveability.
Adjustable handle - a feature that's important for everyone not in the "average height" category - the possibility to adjust the handle height. If you're very short and your partner very tall - there's not even a question about it; it's a must-have for your future stroller. A situation with both parents quite tall also calls for this feature to be present. Almost no strollers are made for "very tall parents only", meaning with a handlebar fixed high up. And if you both are on the short side but think of an average- / mid-sized stroller (system), a handle that can go lower will also be an asset for your everyday riding comfort.
There are two ways that adjustable handles can work:
Firstly, the telescopic mechanism of adjusting the handle - meaning you are pulling the handlebar out to get it higher (and further away from the base of the chassis). This way is more suitable for taller people with longer steps because it eliminates the rear axle's kicking at least partially. On the other hand, the downsides would include a wobbly/almost-breaking feeling with some strollers (it always depends on how sturdily the stroller was made). Another minus would be a change of gravity center, often making the popping up curbs more difficult.
The second method of handle height adjusting would be by rotating it. That means two joints allowing the end of the handle to be turned upwards or downwards. This way creates a significant advantage mainly in the lower-height settings: helping the center of gravity thus the stroller's maneuverability. A disadvantage might be the fact that two joints = two weaker points, and with big loads or pressure on the handle, there's a slight possibility of a joint getting damaged. Remember, this usually happens if mistreating the stroller, not with regular use. Another downside of a rotating adjustment mechanism is that it can be uncomfortable for tall dads - since it doesn't stretch further from the stroller. In fact, in the higher positions, it even goes closer to the base. A tall person could easily start kicking the back of the chassis if not watching out.
Always think of how often the dad/tall person will push the stroller, however, because it may be that it's going to be you 95% of the time. And if that's the case - and, e.g., a rotating handlebar suits you better - the daddy might simply adjust and hold on on those stroller-related occasions.
It is important to find the right handle height for you to steer the pram or pushchair in the most comfortable way possible. Always put the handlebar to the lowest position that is OK with you (you're not kicking the stroller, not hunching over while walking, and so on) - not higher. The lower the handlebar, the easier it is to lift the stroller up a curb or over a hole in the ground.