Also how does property tax get affected by all this?
“Free” months are to prevent advertised rents from being lower to prospective and current tenants. It helps set a higher price anchor so that future negotiations are based off of that.
Obviously norms in different places will vary.
We have not moved a lot as a business, but when we have it's been -expensive-. Leaving aside the cost of actual movers (reasonably low) it costs money to prepare a new office for us.
Granted, we place a premium on employee comfort, so we spend a bit on kitchens etc, but also networking, furniture, the offices themselves and so on.
Then there's down-time for the move itself (packing, moving, unpacking.)
And "rent overlap". Once construction starts we're effectively renting 2 offices, the old one and the new one. In cases where serious work was done this can span 2 to 3 months.
So we've always negotiated "rent free" starter months. This mitigates (some of) the extra expenses incurred. In some cases this translates as discounts for the first year or whatever.
In the other direction, there’s the whole area of “net” rent (and “triple-net” rent), and the bizarre calculation for shared spaces that makes “square foot” a semi-imaginary unit.