IBPS PO and IBPS RRB exam looks for two things in the candidate :-
1. Speed.
2. Accuracy.
So attempting mock test is very important to acquire these traits in actual examination.
When I was preparing I have religiously given Mahendras mock test. I will give you reasons for the same.
o Large number of candidates appearing from all over India so better visibility of ranks
o Paper level of IBPS RRB https://www.eduncle.com/ibps-rrb-recruitment-exam is neither tough nor easy it is as same as actual examination so you will be much familiar with the pattern while attempting actual examination.
o You can thoroughly examine your mistakes after getting ranks and numbers.
o Last and not least I will tell you by my personal experience things like confidence play huge role in impacting your outcome of the exam, while other mock series only presents tough paper, Mahendra mocks test your temperament in any types of paper easy/medium/tough that will slowly boost your confidence.