Just like a milkmaid's three-legged stool, anything supported on three points isn't going to wobble; all three points will always be on the support, even if things aren't quite level. Triangular man-hole covers (usually implemented as two adjacent triangles, as here) therefore don't wobble, and that means they don't go bang if a vehicle drives over them, even if they or their frame are warped, twisted, or uneven. Broads is one maker who capitalised on this property, marketing their covers under the name "Silent Knight". The covers were often customised for particular users; this one has the initials "PO" (for Post Office) so presumably covers a telephone cable junction box of some sort. 

This cover isn't particularly decorative, but some makers went the extra mile and included attractive designs on their covers. Globally there are tens of millions of covers of varying size, shape and purpose. If you're interested, you can take a look at the ultimate manhole covers site...

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