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).
