Why are Bernie Sanders and Tulsi Gabbard still on the ballot?
When he announced the suspension of his campaign on April 8, Sanders said that he would remain on the ballot in states that have not yet held primary elections. While it's unlikely that he would get enough votes to get the nomination, voting for Sanders during the primary would still accrue delegates for the Democratic National Convention (which will likely be held virtually in August).
Sanders said that more delegates in his favor would "help us shape the new platform of the Democratic Party." In other words, it could help push Biden, and the Democratic platform as a whole, further to the left. Biden currently has 1,548 pledged delegates and Sanders has 983.
Honestly, it's unclear why Tulsi Gabbard is still on the ballot, as she dropped out in mid-March and has not made any statements about gaining delegates to push the Democratic platform in a certain direction. Gabbard currently has two delegates.
What happens if I vote for Bernie Sanders (or Tulsi Gabbard) instead of Joe Biden?
Sanders needs 25% of the delegates at the Democratic National Convention in order to be on the three committees at the convention that help shape the party's platform and the structure of upcoming elections. If this sounds vague, it's because it's unclear what issues Sanders delegates could have influence over until the convention actually happens.
But mostly, Sanders (or Gabbard?) getting any significant portion of the vote, despite not being in the race, would be an indication to Biden and the Democratic leadership that those voters are still a demographic worth paying attention to.
If I vote for Sanders or Gabbard in the primary, does that prevent me from voting for Biden in the general election, if he wins?
No. The candidates you vote for in the Democratic primary, presidential or otherwise, do not affect who you are able to vote for in the general election.
So does it really matter who I vote for?
The way the current primary system is structured makes it seems like states that vote later, including Pennsylvania, don't even get a say in the presidential nomination. But voting is a right, and it is your right to choose the person you believe in the most (or hate the least).
Ultimately, you should vote for the candidate you think is the best person to lead the country, whether you think they have already secured the nomination or whether it seems impossible for them to win.