Startups of every variety need cash to operate. Biotech startups often find that they can use shares in their company as a means to acquire cash. Since it takes so very long to develop successful therapies and is so very expensive this process of issuing shares for money often continues again and again.
Each time more shares are issued it creates a solution for the company which needs the cash to operate. It also causes dilution where existing earlier investors in the company find that their percentage interest in the company is declining.
That is not a problem if the company's value is increasing by a commensurate amount. The question that shareholders must ask themselves as this process plays out over time is whether value is increasing as fast as their percentage interest is declining.