The basic explanation is above. If you have an existing database that you'd like to convert you will end up with a multi-value single table within a relational database system. There's nothing against that apart from the fact that you're not making full use of the capabilities of the system. I cannot comment on speed, that is a different topic.
As for relational databases, here is an example I found through Google (
source):
The strength of the system is that you can combine data from different parts of the table structure (eg. Name, food, weight) and create new lists or tables any way you like. The weight/height table could be used to record measurements taken over several years but you would not need to list all the particulars for the dog with every measurement as the tag# links it to the rest of the data. Hope this helps a bit.