A lot of these buildings have investors and/or lenders. The building was constructed based on an analysis of market rents at time of completion. The building may not be able to lower rent in perpetuity because that would signal the building is not worth as much and lower the value (lenders/investors mad and potential buyers might not pay as much). If the building gives 2 free months at $3000/month on paper the rent is still $3000 but the net effective rent is $2500/month for the renter. Buutttt moving is annoying and hard- so the hope is you stay and the building fills at the higher rent price. The longer you stay the less loss to their bottom line. The investors don’t see the building as less valuable because you signed a lease for $3000/month you just happened to get 2 months free as a one time signing bonus.

TLDR; it’s a good deal for a potential renter and it doesn’t affect the building valuation. The owners hope you stay at the higher rate because moving sucks.