asp.net membership issues

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ASP.NET

membership

provider

hi guys, i am new to asp.net membership, i dont kown very exactly it means.

i looked for lots of info and as i kown,i can type "aspnet_regsql.exe" in cmd.exe to create a database with tables(user,role etc). and then we can use custom store procedure
to make the login page, or create, delete user, all is done.
So
1st, i dont kown why we will still use WSAT(web site administration tool) to create role,user etc? just for testing?

2nd, i dont understand why in our web.config file, we use

<membership defaultProvider="AspNetSqlProvider">
        <providers>
          <clear/>
          <add connectionStringName="connectionString"
               name="AspNetSqlProvider"
               applicationName="/"
               type="System.Web.Security.SqlMembershipProvider, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
        </providers>
      </membership>

we have already all the tables after "aspnet_regsql.exe", so why this snippet of code?

does that mean, in our code, we cant use

Membership.CreateUser() 

until we do all these?

thanks for any help!