Even if it is all one class, the labs still have separate meeting times. A 4 hour chem class will have the usual T/Th or M/W/F meeting times for lecture plus a separate lab that could meet at any of 2-3 times any afternoon of the week. The registrations are sometimes set up such that you register for the 4 hr chem class but it isn’t added to your schedule until you select one of the 0 credit hour lab times. It’s no different in terms of how many classes you have to remember to go to.
The biggest difference is that students can’t choose to drop without dropping the whole course. When the classes are separate credits, occasionally a student will drop the 1 credit lab while continuing in the 3 credit lecture (or, rarely, vice versa). This can cause the numbers to get out of sync, since the student will then need to take the other part of the course in a different semester. When it’s all one 4 hour course, the lab grade is usually 10-25% of the lecture grade.