GDP

Why are there two measures of GDP ?

Technically, there are 3 (you would only need to know 2, because the ONS likes to be difficult). According to their website:
"There are three district ways of measuring GDP – output (the goods and services produced in the economy), expenditure (money invested by businesses and spending by households and government) and income (business profits, household income and government tax take). These should theoretically add up to exactly the same number. However, we use a huge number of data sources to measure the economy and the data we receive to do so builds up over time as it all becomes available. This means there are small discrepancies between the three measures but we are always transparent where these discrepancies exist."
https://blog.ons.gov.uk/2022/12/12/what-is-gdp-and-how-do-we-measure-it

In other words, it helps with cross checking since the data can be inaccurate or incomplete.
Even then GDP is an inaccurate and unfair measure of the welfare of an economy, which is why you then get dozens of other different measures that economists use. However, that's one can of worms we can save for another day.