In general there are clients that run on the end user's computer, and servers that run on computers you control. The servers are provided with a secret key so you know they're official executables running on your own deployed machines.
Servers that you control aren't easily compromised. If your servers are updating statistics it is because the game players did exactly what the statistics say they did. (Or because of a bug.)
Game clients can be modified, can be hacked, can be debugged, or in many other was manipulated to send data to PlayFab. Someone monitoring the connection can read the protocol (it's easy enough and uses published REST calls) which is enough to sign on, publish their own values, and post whatever they want. If clients can publish their own numbers, the moment an attacker realizes they can do it they'll set all the scores to INT_MAX or to whatever your game sets as a perfect store. You'll see impossibly high player statistics on anybody who wants them.
As you look more into PlayFab and to many other similar systems, you'll find there are client-specific calls and server-specific calls. Usually servers can read and write anything, but clients can only read a subset and only write a smaller subset.