Well first of all it is important that something like := exists because we need to translate old Perl 5 code into Perl 6.

But beyond that, well if you don't know how to use typeglobs in Perl 5, then you likely won't need to know it in Perl 6. However that does not mean that its job is lessening how many keystrokes to use. Rather think of it as selectively changing which keystrokes to use. Without that ability you couldn't write something like Exporter. Without the ability to produce Explorer you wouldn't be able to easily write something like:

use CGI qw(:standard);
my @parameter_names = param();
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So you see it is quite important that the capability be in the language, even if you will never use it directly.