Almost every tea product I know of uses square tea bags.
Except Tetleys.
Tetley teabags are round.
Wouldn’t that mean wastage in the teabag fabric?
Square teabag, little wastage. Cut on the square, nothing in-between wasted.
Round teabags, doesn’t that inevitable mean wastage of the teabag fabric?
Inquiring minds!
From the source
I’ve been given tea from a company called Spot. Their tea is in what looks like mesh cloth pyramids. I assume it’s because it looks cool.
My guess is that they just do it to set themselves apart from other brands and be distinctive in some way. If there were a real tangible benefit to round tea bags, other companies would do it too - any patent on round tea bags that might possibly have existed at some stage would surely have expired by now.
Round and tetrahedral bags are becoming more common. Ooh, the fights between nylon vs silk tetra-bag aficionados!
Of course, us (smug arse) loose-leaf tea drinkers are above such petty concerns 
What’s that? Pot vs infuser, you say? Well, look at the time…
Moved to Cafe Society (from FQ).
Here in the UK round tea bags are probably slightly more common than square - many of the supermarket own brand teas are in round bags, as well as other brands. I remember when Tetley introduced them in the late-80s/early-90s, one of the advantages given was they fit better in the cup/mug and thus more of the tea can make contact with the water. 
OB
I know that the Marks and Spencer Extra Strong that I have shipped to New Jersey by a little shop in Kent has round bags. They also lack the strings and tags with which American teabags can be removed and discarded without needing a spoon.
Republic of Tea and Typhoo use round tea bags. PG Tips and Tea Pigs use pyramids (supposedly better flow). Lipton has had the Flo-through tea bag for decades.
Wastage of the fabric hardly seems to be a concern.
Interesting. Thanks all.
Tetley introduced round teabags some years ago. Although my memory is pretty good, once in a while it takes me time to recall important things. But fortunately, I can remember television commercials over many years verbatim. It probably isn’t the optimal use of scarce brain space.
The commercial Tetley made after introducing round teabags featured the song I Get Around and derided the competition - “The others are square.” I’m willing to believe all of this effluvium is marketing flotsam. But they also claimed the high teabag technology increased saturation, offering consumers “taste to spare”. They basically did it to look cool (such as that goes in the teabag world), use a specific song, and differentiate themselves. You’ll have to argue with @MrDibble about rooiboos and other shapes. Let the fireworks begin! (Pops corn. Thinks better of it and does something productive instead.)
There is a comparison with Wendy’s somewhere here, which trumpets square, never-frozen meat patties.
MatPat did a video on his Food Theory channel testing various ways of making a cup of tea.
Here is a link to the video if you’re interested. If you open the description below the video, they give time stamps for “key moments” as the video is about 21 minutes long. At 5:52 they list what they say is the best material and shape for a tea bag, what order to add tea, milk etc to the cup, what size shape of cup and so forth.
I will let yall be your own judge of his methods of testing.