@dzxm,
There is a lot to this topic and it has been brought up many times. The net is that you can get an external antenna which can make a difference in your device's cellular performance. Better signals make everything better - less data, less disconnections, better battery life and easier updates / diagnostics.
That said, it comes with a big price which is in certification. Particle has a certification for the device / antenna combo and if you change the antenna, you invalidate the certifications. So, I am hoping that the on-going improvement of cellular coverage, Particle's improving EtherSIM options and better radios / antenna - and now LoRA for reaching into dead spots will make external cellular antennae unnecessary.
That said, when there is no other choice, I use the Taoglas (Taoglas TLS.01.305111) external antenna below with a proper ground plane and as much elevation as possible. In those cases, it can make a big difference.
Here is a thread where this antenna is discussed:
and here is an example of what elevation can look like:
Hope this helps, Chip