Yes exactly. AA calls it Main Cabin Extra, AS calls it Premium Class, UA calls it Economy Plus, DL calls it Comfort Plus, but it's basically the same. You get extra legroom and maybe a free drink (no free drinks on UA). On long domestic flights (like transcons) these can cost $100 or more as a seat selection fee; on shorter flights they are less but still not cheap.

If I'm flying on an airline without status I usually try to book first or business if the cost is reasonable, but if it isn't I'll book economy and pay extra for seat selection to get the extra space. Paying for this every time really adds up; status passengers generally get this for free.

There are also significant savings from having free checked bags on every flight.