Cross Site Scripting is Number 3 in the OWASP Top 10, making it one of the more serious security vulnerabilities. It’s easy to exploit, very common, and can do a lot of damage to systems and businesses. It’s important to protect against it.
Some user interfaces will filter out some types of XSS attacks, but not others, so it is worth trying different inputs, so to evade filters.
Resources and further learning: