Sitting in an ergonomic chair feels very supportive and balanced and noticeably more comfortable because you are sitting especially for long hours. It gently guides your body toward a healthier posture. You will not feel quite so stiff or so restricted because of that.
A regular chair often forces slouching or shifting around for comfort unlike an ergonomic chair, encouraging natural alignment while it cushions your body. A supported back, resting arms, also minimal adjustments toward comfort result in a sense of ease.
It feels like your body is being held in the right position

An ergonomic chair supports your lower back by aligning your spine and keeping your posture upright naturally. You feel stable from head to toe plus pressure points are reduced because of that.
It is likely that you will notice that the lumbar support fits into your lower back the moment that you sit down. It keeps your back in a gently upright position because it does not force it to be. Hips level, feet flat resting on the floor, and shoulders less hunched happen. Usually, the seat feels padded enough to stop fatigue, but hard enough to keep you upright.
This feeling of proper alignment can be subtle to detect at first. After sitting for a full day, the difference becomes obvious. Your back will not feel as tense, and soreness or discomfort will not distract your focus.
It becomes more comfortable the longer you sit

While a regular chair might feel fine for just the first hour, it often leads to some shifting if you are fidgeting or even slouching. Even the hours do pass, an ergonomic chair stays comfortable.
The chair gives support for your body evenly. Because of this, pressure on any single area is lessened. That means that your legs feel a bit less numb, and your shoulders cramp to a lesser degree, and you are much more stable throughout the day. You stay more focused so relaxed throughout work or study because you’re not constantly repositioning.
Many people describe sitting in an ergonomic chair as “balanced” or “weightless”. It simply holds you in just the right place it doesn’t pull you either in or push you further forward. Adjusting to that feeling may require time. It is difficult returning to a normal chair then.