I don’t think Pope Francis is seeking to “use” Sanders nor does the Pope need any platform that Sanders would or could provide.
That said, Sanders would love to use any invitation or visitation with the Pope to further his own political agenda because Sanders, like many, is under a misconception that the Pope can change Catholic doctrine and dogma though the two share a similar distaste for free-market capitalism. Sanders would prefer capitalism’s demise whereas Francis would prefer restrictions that limit greed and provide for some equity.
Another reason for Sanders to leverage a meeting or handshake with the Pope is to court so-called liberal Catholics, though I would argue that Catholic supporters of Sanders do so in contradiction to Church teachings on matters of social justice, life, sexuality, etc.
Bernie Sanders’ pursuit of Pope Francis is one of pragmatism and convenience. Nothing more. Once Sanders realizes that Francis remains Catholic and cannot affect change convenient to Sanders, that desire to affiliate will diminish.