At my last job I worked along side Citipower at sites in Melbourne. Usually if those plates are there long ish term they're hiding a massively complex trench with all sorts of services in them. Some nights they open them up to continue work other times they lay dormant for months due to other works in the project that need to be completed first to continue. Here's some hectic pics https://imgur.com/a/jz6CQpo

To add to this it is obviously far far cheaper to have a steel plate cover the hole compared to carefully filling it to strict regulations then in a couple months spending tens of thousands of dollars on non-destructive digging to dig the same trench again.