ForceNetUpdate means you are forcing the system to replicate the data on the next frame. Reliable events don’t replicate immediately until the next NetUpdateTime tick.
The advantage of ForceNetUpdate is you can lower the NetUpdateTime so you don’t waste much CPU cycles due to checking the state of the object has changed every NetUpdateTime tick.
Say you have a Door that only replicates when it is opened or closed. It is a good thing to either lower its NetUpdateTime or put it on a NetDormancy and then just call ForceNetUpdate when it is either opened or closed.
If you have a ParticleFX that you want to sync between clients every, say 5 seconds, then Reliable event with NetUpdateTime of 5 seconds is a good way to do so.