The only move suggested here by every chess engine is
Can anyone please advise why is it so important?
[fen "r3k2r/ppp2pp1/7p/3pP3/3P2b1/5N2/RP1KBPPP/7R w - - 0 1"]
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Black is down material but still has some strength. The most obvious one is that he has a pawn majority on the queenside, meaning potentially passed pawn if he manages to push his majority. So Black's plan will be to put both rooks behind the pawns and go for ...a5-a4, ...b5-b4 etc...
In a very simple way White can fight against Black play (it's prophylaxie) by pushing b4 preventing ...a5 and ...c5. Moreover if we manage to push b4-b5, the pawn on a7 will now be a weakness so we can attack it (doubling rooks on a-file).