Tab top curtain panels are a form of window dressing that is looped through a curtain rod instead of using hooks and a curtain rod with rings. They can be thick and opaque or soft and sheer. There are several reasons why tab tops are preferable to using window curtains with hooks.
The first reason is that curtain panels with tab tops are easier to make. All you have to do is cut the fabric, hem all round, and add a few loops to the top of the panel. Regular curtains have the additional step of choosing and sewing on a curtain header and these can be quite complex especially when it comes to working out how much fabric you require. In any case, it is always a multiple of the width of the window as the curtains are gathered at the top.
This leads to the second reasons why tab tops are better. You need less fabric as tab tops do not need much width at all and are generally straight rather than gathered when they are drawn across the window.
Fitting is easier too. The loops from the tab tops are simply threaded through the curtain rod. Pleated or gathered curtains, on the other hand, have to first be loaded with curtain hooks. Next, the curtain rail has to be filled with rings, the right number for the number of pleats or for the width of the curtains. The curtain hooks then have to be individually slipped into the rings. Each time the curtain is drawn, the hooks may slip out of position, and fall out. You then have to reattach to reattach the curtain. Tab tops never have this problem, because the loops of fabric hold the curtain firmly on the rod. Looping the panels through the rod makes it hang straight down. Regular curtains, with their system of hooks and rings that sometimes slip or come loose, often drape askew.
Tab tops are quite simple which makes them particularly suited to contemporary rooms. Anything gathered or pleated is not going to work so well in an ultra-modern apartment or home. If you have an older house or more traditional decor, you can still use tab tops, however provided that the fabric is traditional in style.