Actually you're a little off there king. Washington state had a caucus system up till now.

Delegates from each of the respective parties met at these caucuses to decide which of the declared candidates would represent them in the general election.

Primaries are a way for the voting public to have a say as to which candidate should represent their party, instead of just leaving it up to appointed delegates.

The reason that you have to declare a party affiliation to participate in the primary election is so that each party decides it's own candidate and cannot influence the selection of any other parties' representative. It is also a way to help streamline the election process and save WA taxpayers a bit of money.

Like you said above though... in the general election you can vote for whichever candidate you like, regardless of party affiliation.