Sometimes your professional body of work can be inadvertently split up because:
Your name changes, e.g. a maiden name to a married name.
Your name is listed differently, e.g. John Humphrey --> John J. Humphrey.
The institution that you work at changes.
In these cases, and others, databases can mistakenly attribute your work as being by two different authors.
What this means for you is a potential loss of credit for your hard work and research. Your H-index (how well you are doing as a published author) is affected adversely because not all your articles are being taken into account.
"ORCID provides a persistent digital identifier (an ORCID id) that you own and control, and that distinguishes you from every other researcher."
In essence, ORCID gives you a unique, digital, fingerprint that you can attach to all your work to, making sure that it is always correctly assigned to you.