To me, it's just a simple matter of whether you want to listen to said celebrity. Be rational. Do you think they are genuine in their beliefs? Do you think they are selling to you, or are they educating you? Are you agreeing because you like their latest piece of media/their public persona or because you genuinely relate to their impassioned speech? Celebrity & politics, for better or worse, have always gone hand in hand. Shirley Temple, Jane Fonda, Clint Eastwood, Sonny Bono, Roseanne Barr, Arnold Schwarzenegger. Hell, Reagan and Trump. Before I continue, I will note that while it is the conservative celebrities who tend to be more successful in their endeavours for public office, it is a far more common phenomena in the left due to the overwhelming liberalism in Celebrity Culture today. Anyway, it's the allure of fame/wealth and the perception that they're still more human, relatable, decent than the politicians (just for the point of the thread, politicians being a catch-all term for any & all representatives) that they're speaking for which makes celebrity so popular in the political field. That, of course, makes them very useful for those with all kinds of agendas to push.
I adore Eva Longoria - I think she's a lovely woman both on & off screen, yet I find it confusing how she has managed to speak at two successive Democratic National Conventions. By all means, I think she's an intelligent woman and has genuine belief in her words. Her public persona is fantastic. But was she asked to be there by the Powers That Be for any reason other than to use that public persona to appeal to certain demographics? She was not. This goes the exact same for Clint Eastwood - his presence at the RNC in 2012 was to sell his persona to a particular demographic. Though, to be fair, Clint Eastwood does have some measurable political experience compared to Eva Longoria.
Does that mean Ms Longoria or Mr Eastwood don't have a place on the political stage? That's up for the individual to decide. All I can say is be wary of what is being presented, do your research and as much as you may love what they've contributed to the world outside of politics, don't let it cloud your judgement.