Questions about prepositions and relative pronouns

The use of "which" is fine, but the sentence has problems - you do not need "currently" with the present perfect tense - it is included in the verbal idea.,

"The only reason for which I have bought this laptop is because I have been travelling for the company I work for."

And here is it people FROM our branches or OF our branches?
2- I'm doing presentations and teaching people from our branches

If they get the presentation while in their offices in the branches , use "in". If they are going elsewhere for it use "from"

You can also use
"The only reason  I bought this laptop is because I travel for work,."

"The only reason that  ("that" is an optional extra here) I bought this laptop is because I am travelling for work,."

And here is it people FROM our branches or OF our branches?  2- I'm doing presentations and teaching people from our branches

I'm doing presentations and teaching people from our branches  This i s good if people are travelling to meet you to be taught. If you are going to their branches and teaching them there,  you might say:

I'm doing presentations and teaching people at (or "in" - either is OK) our branches.

Questions about prepositions and relative pronouns

The first sentence is okay, but you could rephrase it as:  "The only reason why I bought this laptop is because ..."

The second sentence:  "from our branches" sounds better to me.