Why do some LEGO City vehicles have tiles in their chassis?
I asked one of the set designers at LEGO, Samuel Johnson, on Twitter. Here is our conversation:

This to aid in the building flow for children aged 5 and up. Easy to spot on the page, easy to find in the pile of bricks :)
— https://twitter.com/Spider_Sam_LEGO/status/1037914372897681409



Ah! Do both the colour and the shape (studless vs studded) contribute to this?
— https://twitter.com/NOVALISTIC/status/1037914613847740416



Depends on the model :)
5year olds aren’t very strong, so it can make it easier to build
— https://twitter.com/Spider_Sam_LEGO/status/1037967767847227392



That makes sense. Too many stud-and-tube connections can make building difficult even for me at times. Thanks for answering!
— https://twitter.com/NOVALISTIC/status/1037994518295994368

So, it's an interaction of several factors including ease of following the instructions, as well as ease of physically attaching elements together (and, presumably in turn, disassembling them).