I would think you would need quotations and concatenation but if you say its working somehow just go with it. If it aint broke don't fix it right?

That method will work... until it doesn't. :)

Under what circumstances would this fail? The syntax is so much easier than all the "" and &s.

It's not supposed to work, some combination of situations is allowing it to read what you're telling it to do like that, but any kind of outside poke could bring the walls down. Whether that be changing a word/field or even something outside of access like a windows or office update could knock some sense into it and make it start to error. It's not long term reliable.

It will sometimes work if you say:

CustomerID = DLookup("CustomerID","CustomerT","OrderID=OrderID")

Until Access can't figure out what you mean by "OrderID=OrderID". If it can find the OrderID field on the current form, then you're usually golden. However, sometimes, it can't. I don't have an exact when/where/how/why for this, but in my nearly 30 years of experience with Access, I know it's almost always better to say:

CustomerID = DLookup("CustomerID","CustomerT","OrderID=" & OrderID)

So that you EXPLICITLY send DLookup the value you want and not a field name. The same goes for SQL statements. You're sending field names, not values. So, like I said, your code will work... until it doesn't. :)