The Democratic primary is very important this year, because it gives US voters the opportunity to choose between a New Deal Democrat (Sanders) and an Eisenhower Republican (Clinton). This is the first time since George McGovern ran in 1972 that such a choice has been offered to the US people.
The Republican party itself no longer offers serious candidates for president (and increasingly often doesn’t for other races either), so the Democratic party has taken over most of the Republican policies and positions and offers Republican candidates a place to run for office without having to associate with the clowns who have made the GOP into a circus sideshow.
Personally, I’m a progressive Democrat, so I strongly favor Sanders’s policies, but I can see a lot of Republicans voting for Clinton, whose stand on almost every issue is a late 1950s Republican position (except that Eisenhower warned us about the military-industrial complex, while Clinton supports it whole-heartedly).
Because the Republican Party is incapable this year of running a serious Presidential candidate, the Democratic primary is doubly important—it is the only chance voters will get to choose between a classic Democrat and a classic Republican.