The first and most important reason for getting the figures right is so that you don't end up paying more tax than you need to. If you get your income figures wrong then you will need to pay tax based on an income you didn't receive. Similarly, if you get the tax figure wrong and report that you've had less tax deducted from you than was actually deducted, you will need to pay the tax again to make up the difference.
Here is a worst-case scenario: if you accidentally said you earned £5000 more than you really did, and you also accidentally said you paid £1000 less tax than you really did, these errors would cost you £2000 pounds.
Secondly, HMRC have records of your income and tax deductions through employment and self-employment, and they compare that with the information you submit on your return. When they spot the error, they will either delay any repayment to you whilst the error is corrected (which can take months!), or if they recognise the error after the repayment has been issued, they will open up a full investigation into your return. If this happens, they will want accurate records of everything, which can be a painful and time consuming process.
Thirdly, if you are looking for a mortgage in the next few years, then getting your income figures wrong could have a significant impact on the amount you are able to borrow.
Let's make sure we get this right.