Tax Adjusted Trading Losses for individuals – Opening years loss relief

Dear Tutor,

First of all, thanks for your amazing videos. I appreciate your passion and dedication to render this course as easy to understand as possible.

I have some doubts regarding opening years loss relief that I will illustrate through this example.

Assume that 3 years prior the start of trading , Sam had a total income of £40,000 p.a ( say it is only property income). Therefore, the taxable income after deducting the PA is £ 28,150 = ( £40,000 – £ 11,850). So the tax payable is ( £ 28,150 @ 20%) = 5,630 p.a prior the begining of trading. I believe that tax payable has already been settled. Now, when it comes to the first year of trading , Sam made a loss of £40,000 and decided to set it off against total income of the first of the 3 years prior trading ( £40,000).
This results to nil taxable income and to wastage of PA.

But what is the rationale behind claiming opening years loss relief as we know Sam has already paid the taxes prior the start of trading ? Does the relief allows Sam to claim back the tax already paid to HMRC ?

Thanks for your help