Two main reasons to it:
1. CyberArk works as a password vault and hence it can securely store the credentials used by Solarwinds (user name + password)
2. CyberArk comes with certain level of automation as well which can sync/change the password on the tool (for service accounts where there is a restriction to change the password every 'X' days, this feature would come into play where CyberArk automation can be used, if Solarwinds can expose a API or something for automatic password change/sync on Solarwinds tool (credentials section like Windows Cred or SMTP Cred or ....) without any downtime on the tool or without affecting monitoring of devices performed by Solarwinds (without skipping a poll) then it would be a cool feature.
I have seen few idea's in the past with respect to the same: