The most obvious reason on the 737 is to check that in the case of an engine failure you can feed fuel from the dead side to the live engine. If you cannot it is a no go item. Marking this in the TLB is a means of making sure people carry out the check for example on extended overwater flights. In Europe there are not many sectors where this would be critical so my company does not require it. If you are in Russia maybe it is different.