Posted confession times in many parishes are pretty infrequent and brief. Sometimes there is only half an hour or so once a week, with one priest hearing confessions. If there are more than half a dozen penitents, it can get pretty cramped for time. There was a time that confessional times were available nearly daily, but in most places, that went out with muscle cars and 29 cent per gallon gasoline.
There’s nothing that says you couldn’t just pop into the confessional for some brief spiritual advice, but if it’s more in-depth than that, it’s really more considerate of your fellow parishioners to make an appointment with a priest for a more comprehensive conferencing session. Sometimes, especially during Lent, before First Fridays, and during the last week or so of Advent, confession lines are long and some people might not even get a chance to confess if there is someone with a particularly long claim on Father’s time. Most priests are very happy to hear your confession before, during, or after, such a scheduled conference, and yes, it can be a mixed session like that.