Can I just buy a universal PWM fan hub and then connect all Case fans into the 4-pin connectors
and then connect the fan hub to the motherboards 4 pin header
No. First, it is important to understand there is a difference between a "fan hub" and a "fan controller". If you just want to add fans, you get a hub. If you want to add fans
and control speeds too, you get a fan controller.
Power is another problem. You don't want to demand too much current through a single motherboard fan connector. I would not attempt to power more than 2 fans through a single connector. For example, I would advise against using a 3-way splitter, or daisy chaining Y-splitters. So you must make sure any hub or controller you get supplies its own power. As kapone32 suggested above, this is typically done through a separate molex or SATA power connector directly from the PSU.
Note that some controllers do let you power and control the CPU fan too. However, if you go that route, you may have to disable the CPU Fan monitoring and alarm features in your BIOS Setup Menu. Some systems will automatically shutdown (or prevent boot/fail POST) if no CPU fan is detected or 0RPM is sensed.
For more information about hubs and controllers, and to see some recommended devices, see
Best Fan Controllers and Fan Hubs for PC.
I will never understad why buy a fan hub when all mobos come with several fan ports for direct connection
I will never understand why people say "I will never understand".
It seems more like a refusal to accept any other possible scenario rather than being intellectually incapable of understanding.
Even for the intellectually challenged, things are typically easy to understand once all the facts are first and foremost,
accepted, and then understood.
So first, it is important to accept that
not "all" motherboards come with several fan ports! In fact, many budget boards come with just two and one is for the CPU. Many ITX and µATX motherboards, come with just 2 fan connectors (again with 1 being for the CPU). Case in point, the
ASUS ROG STRIX B250G Gaming Micro-ATX board.
Some people are concerned about looks and cable management. Some fans come with short power leads. Some larger cases may put the case fan far away from the motherboard fan header. A hub/controller may solve the distance and cable management problem too.
Some folks are very concerned about fan noise. This is particularly true with HTPCs (home theater PCs). A fan controller may allow multiple fans to run at slower speeds in near silence and still move massive amounts of cool air where a single case fan would have to spin at faster (and louder) speeds.
No doubt there are other reasons some may want or need a hub/controller. Having an open mind to additional possibilities can help one understand why someone might do something too.