On a bike that you ride around the block once a month a fit is irrelevant. On a bike that you ride 15-25 miles a day, and 50+ on weekends, it's critical for avoiding unnecessary pain, strain, and even injury.
Factors that contribute to the need for a new fitting: Age, fitness level, bike geometry, your level of demand for efficiency and/or comfort, distance ridden, frequency of rides, riding style. When I ride a lot, the difference of a half centimeter of cleat placement or saddle position can make a difference in how my knees tolerate the repetitive stress. And my fit changes a little as my fitness ebbs and flows. But I could jump on my kids' bikes and tool up and down the street for five minutes and fit wouldn't matter at all.
Last edited by daoswald; 01-03-20 at 01:49 PM.