MRR is the monthly equivalent of ARR. Annual contracts are divided by 12 to compute their monthly contribution. MRR is favored by month-to-month SaaS businesses (consumer SaaS, freelancer tools, prosumer products) where the monthly cadence aligns with how customers actually buy and pay.
MRR decomposition into 'new MRR', 'expansion MRR', 'contraction MRR', and 'churn MRR' is the standard SaaS health diagnostic. Net new MRR each month is the leading indicator most operators track.