Why did century ago  use skeleton keys and does old locks?

Would you like to try asking that in a language that Earthlings can understand? 

Because skeleton keys making good keys for old locks and does fit in with keys in the locks many century ago. Hope this helping.

we actually STILL use them occasionally.

there were many uses for such simple locking systems. to keep children out of things when parents were not around, seems to be most popular.

also, doors were not as well built back then as they are today. there was no real set of standards dictating common doorway sizes, and measuring devices were often inaccurate. so doors did not always stay shut when shut. and the latch systems of that day were still very basic and coarse, and were not as good at keeping a loosely hung door closed.

also, for pantries, cabinets,  and such storage areas, there were no magnets or clasps to keep doors shut like we have now. so they just locked it, and a skeleton key, often held by one or both homeowners, was used to gain access when needed.

the waitstaff/servants of a home might each have one as well.

in some businesses, a skeleton key was used by cleaning & janitorial staff to get access to every room for cleaning.

however, you don't use the traditional skeleton keys you see in photos for that. those large iron skeleton keys were only used, not to stop criminals. just to keep things closed until they needed opened. it was only for infrequently accessed areas.