Hello Pietro;
I guess what we are trying to find out is "what would you want to do with an unprotected SFB"? Study the code? Copy or modify the block?
SFB/SFCs are standard program blocks that fulfill one specialized function each, the same in any S7 CPU (if they are loaded in the CPUs ROM, nao all CPUs can use all SFBs). If they were modified by someone, you would no longer have uniform applications of the logic, and that could lead to complications. Descriptions and explanations for each of the SFB/SFCs are available in the "System and standard functions" pdf manual; it is available in the Simatic/Documentation folder of the computer where you installed step 7; it is also downloadable from Siemens' support website.
On the reverse, if you do not like the way an instruction reacts, create your own (as a FB or FC), use yous SPROT executable to lock it, and re-use it in all your next projects. The logic will always execute the way it was programmed by you; no one else can modify it (unless they can unlock it....). We have people here who have created timers that operate "better" than the ones provided by Siemens, or PID logic that suits their needs.
Hope this helps,
Daniel Chartier