Without number or name plates, it was hard to tell locos apart when they went past you at any speed.
The plate identified the locomotive, it gave it an identity. Some parts might change, but the number plate always stayed the same.
The number plates always stayed with the locomotive. Even when the loco had a whole new cab fitted. When I worked there they were always painted red and highly polished.
Did you see the 15f locomotive in George Square in 2008? Do you remember seeing earlier locomotives in George Square? Do you remember traveling on steam trains?
On holiday in South Africa, I was walking round Johannesburg station and found a shop/museum selling railway memorabilia. On enquiring if they had any North British plates, the shop owner told me that 3007 was one and so I bought it. My wife was not best pleased when she saw it and said that the airline might not let it on the plane as hand luggage, so I ended up putting it in the suitcase. Boy was that case heavy.