The Seed Enterprise Investment Scheme (SEIS) and Enterprise Investment Scheme (EIS) are UK government schemes designed to help smaller higher-risk trading companies raise finance by offering a range of tax relief to investors who purchase new shares in those companies. They aim to make investing in startup business more attractive to investors by mitigating some of the financial risks. They have a number of rules in place that the company must adhere to before they can quality for the schemes. This is based on the kind of company it is, the amount of money it wishes to raise and what the money will be used for.
You don't have to offer EIS or SEIS, but if you don't it's unlikely that you'll make it onto Crowdcube, as tax relief is very important to investors.
You can find a great video by The School for Startups explaining the EIS/SEIS process here