Question

Why do we convert measurements

Answer 1

you convert measurements because it would be really helpful to know them in other types of measure, example: it would help if you knew that you needed 500cm of fake grass your garden, but a store only sells fake grass in metres. You would have to convert from 500cm to metres, which would obviously be 5m.

Answer 2

You may understand this considering two examples:
1] If they announce that your train is arriving in 5 you may be tempted to go for a coffee...but is it 5 minutes or 5 hours? This helps you to see that the same unit of measurement needs to be used differently to adapt to the circumstance. If we had only, say, seconds you 'd have instead of 5 hours...18000 seconds which is not very practical!!!
2] if you want to buy milk you ask for a liter or pint because you need a certain Volume of it....if you go to Italy and ask for pint...it doesn't work...you need to change it into liters...otherwise....no milk in your tea!

Finally; sometimes you need to convert to deal with small or big quantities, other times you have to "bridge" between different ways of measuring the ame thing (due to culture, tradition, availability, practicity...etc).