Basically, manholes, which serve as the entry point for those who clean underground sewerage pipes, are located at every major junction of sewer lines. With so many manholes around, the chances of a person falling into them cannot be ruled out; and hence they have to be neatly covered. But why do we always get to see round manhole covers?
One of the most obvious reasons for the manhole covers to be round - as anybody would guess it, is to facilitate easy transportation of these covers from one place to another as they can be easily rolled by one person - something which is not possible with square manhole covers.
Round shape also ensures that they don't fall in through the circular opening. (If the manhole cover shape is square, it can fall inside when inserted diagonally.) This can be attributed to the fact that the lip which is designed to hold the cover reduces the diameter of the hole by an inch or so, and allows the cover to settle.
Again, round manhole covers do not have to be aligned when putting them in place; they will fit in easily irrespective of how you place them. The same rule applies for septic tank covers as well.
At the same time, the shape of the hole has to be taken into consideration as well. The manhole is given a round shape as this shape makes it hold on against the compression of the earth from all sides. Digging a round hole is much easier as compared to digging a square hole. At the end of the day, when a hole is round, the cover is also round.
What about the Reuleaux Triangle?
One may argue that a Reuleaux triangle - wherein all diameters are of the same length, would also serve the purpose. But then manufacturing round manhole covers is easier than manufacturing Reuleaux triangular, as pouring hot metal in a circular mold and allowing it to settle is easier than doing the same in a mold of some other shape.
Initially, it was believed that equilateral triangles are also ideal for manhole covers, but the same was eventually proved wrong by researchers. (Interestingly, the city of Nashua in New Hampshire happens to be the only city in the United States to have triangular manhole covers, all of which point towards the direction towards which the sewage is flowing).
All these logical reasons hint at the fact that this question about the round shape of manhole covers is not some why-did-the-chicken-cross-the-road question, but is in fact an aptitude based question which you might face the next time you go for an interview. So, now you know the logic behind the circular manhole cover.