Fitness

It's definitely not required but people tend to dedicate a whole workout to the larger muscle groups for the sake of structure and focus.

Your legs have many large muscles so they require a lot of work and a variety of excersise. Combining these excises into a single workout helps ensure you target all of those muscles.

It also allows you to do compound excersise like squats followed up immediately by targeted excersise of individual muscles which I personally think is more efficient than doing them on different days.

It really depends on what type of program / split you go with. Are you following a program or just winging it? Generally speaking You are much better off with an established program that sorts this all out for you.

I used to do splits. Now I do full body variation every 3 days instead of every other day. Even though my workouts themselves are longer, the extra rest is worth it, as well as more time to do what you want on those 2 days off. Usually some type of core and cardio. Learned this program from Greg Doucette on YT. Great for older gym rats.

I’d say you should put some focus into leg days. I do once a week.

I do biceps on back day since those muscles are often used anyways so I do pull downs, superset of rows/bicep curls, superset pull downs/hammer curls, face pulls and usually hit shoulders after that.

I do triceps on chest day since I’m similarly working those muscles anyways.

It’s good to shake things up periodically too so don’t lock yourself into one program indefinitely. Be open to adding/removing or rotating new things to keep your muscles guessing and avoid imbalances.

It's good to have a routine that targets a rounded balance of muscles, particularly stabilising ones to avoid injury.

It's actually counterproductive to do leg day vs arm day in theory, as the more tired you get the less effective other exercises are going to be and the higher the risk of injury.

In general though because we have lives to plan around, and often cardio or like a job which heavily relies on legs it kinda sucks to be doing that on tired legs all the time.

So while biologically less productive than splitting up different muscle groups through the whole body in practical terms it's often easier to practice.

Hell, some people split legs into quad and ham centric days, with good results.

Legs definitely deserves it's own day given how much work they can do. I personally just tack arms onto the end of my chest day, which is at the very end of my four day split.