It matters because it is the number a buyer values and the number that pays your fixed costs. Two firms with identical annual revenue are worth very different amounts if one earns it from contracts that renew and the other from projects that must be won again every quarter. MRR is also what tells you whether growth is real: adding clients while MRR per client falls means you are buying revenue with margin.
The common mistake is counting anything that arrives monthly. A twelve-month payment plan on a hardware purchase is instalments, not recurring revenue — it stops when the hardware is paid off, and treating it as MRR overstates both the run rate and the valuation.