There are useful cases for this. It's much less ambiguous in those cases if your language allows the definition of the variable in the same location, and scopes it. For example, Perl:

    use strict;
    use warnings;
    
    sub one { 1 }
    
    if ( my $one = one() ) {
        say $one; # prints 1
    }
    
    say $one; # compilation error, "Global symbol "$one" requires explicit package name"

This case is trivial, but when you want a temporary variable and don't want to clutter your scope, it can be useful.

That's not to say assignment and equivalence might not be better off with different operators, as noted by others here.