Do not get overeager
Though adding a gorgeous illustration might seem like a quick, simple fix to many design problems, you should be very strategic and thoughtful. Overusing illustrations can oversaturate your users, ultimately causing them to end up ignoring all your hard work. For maximum impact, only use illustrations in key moments and situations.
Having custom illustrations can help shape and define the feeling users get when using and thinking about your product. They can add life and soul to an otherwise very mechanic process of shopping online, making healthcare appointments, managing your finances, etc. Be strategic when working with an illustrator or creating illustrations yourself, and plan out the key moments in your product that would benefit the user most. Last but not least, create something cool, memorable, and unique to your product.