Too many rules
The main thing that IR35 and the other rules have in common is that they were created to prevent individuals from avoiding PAYE tax and NI contributions.

Why does it matter to HMRC?
The answer to the question is tax and NI avoidance. Employment income is liable to PAYE tax and both employees’ and employers’ NI contributions but if you can change the character of income from employment to profits from business you can reduce or avoid PAYE and employment-related NI and replace them with lower NI, and in some instances lower tax alternatives, e.g. dividends.

Why does it matter to you?
Whether any of HMRC’s anti-avoidance rules apply and who is responsible for deciding affects the net income you receive from a contract. If you don’t know that before you sign on the dotted line you might get a nasty shock when you receive your first payment from a customer. Plus, there are different (and often fiddly) bookkeeping issues where IR35 , off-payroll or agency rules are involved. You may therefore decide to use a professional bookkeeper rather than handle the task yourself. Finally, there’s a risk of double taxation where any of the rules apply. Preventing this requires special tax elections

The net income you’ll derive from a contract relies on the specific rules that are applicable. These rules can influence the pricing and other terms you negotiate for the work. Moreover, the responsibility for determining and adhering to these rules varies depending on which ones apply, and this, in turn, affects the corresponding bookkeeping obligations.
Before entering into a work contract, ensure you are aware of the applicable rules. If the contract falls under agency or off-payroll rules, it’s the responsibility of your customer or the agency to determine this. In such cases, they will deduct PAYE tax and National Insurance (NI) from the fees paid to your intermediary for the work you perform. However, if neither of these rules applies, the decision regarding whether IR35 regulations are applicable to the contract rests with you.