some doubts about nodeblock in *.tim

[dr]

Dear all ,
Currently, I'm studying  the format of *.tip and *.tim. I found that, in the 
*.tim it not only stores suffixes information, but it also stores information 
of the  current prefix's child nodes' position.Taks terms 'ab1,abc1,abc2‘ for 
example, for the prefix 'ab', *.tim stores the suffix '1' and the file position 
of terms starting with prefix 'abc'. I wonder why, because the file position of 
prefix 'abc' can be got form FST,  and prefix 'ab' knows the existence of 
'abc',why we store it in the *.tim again? For the speedup of WildCardQuery?
The version is 4.10.3.


Thanks.