Linked from
The 14 pages that link to Parker Solar Probe, each with the reason it gives.
Stellar windRelated: Its close passes sample the wind near its source.
Solar windRelated: Its close passes measure solar wind near its source.
Solar coronaRelated: Its repeated close passes sample the corona and its escaping plasma.
Sungrazing cometRelated: Its measurements probe the environment that sungrazers traverse.