For two reasons:
1. if I have 100 equations and I want to cross reference them, then I have to first jump to the equation and find out what is the number of its caption e.g. 37 or 59 and then again get back to where I was writing. If I put a bookmark on the caption, then I can put a name i.e. force_relation and I can find it in the list of all bookmarks quickly.

2. I found that if I want to use cross referencing for two or more captions like:
... as shown in Figures 1 and 2 ...

then I have change the field code for those captions to { REF _Ref218925266 \# 0 \h }. Since I am not sure which of the caption would be called in the remaining of my text, then I have to change them all to get rid of the text before and after captions (label and caption).

so I prefer to put bookmark on all captions and specify a name to them all. If I am doing wrong please let me know what is the more logical way to do it. and if what I am doing is right, please specify how I can find a field in the current line or paragraph or table and put a bookmark on it.