This simple helper-function lets you replace commonly seen input-argument handling code with shorter and clearer alternatives. For example, instead of:
if nargin < 2
x = 42;
end
if ~isfield(options, 'verbose')
options.verbose = false;
end
if ~isfield(options, 'method')
options.method = 1;
end

you can simply have:
default('x', 42)
default(options, 'verbose', false)
default(options, 'method', 1)

By avoiding nargin, you can more easily change function definitions without messing up all of the argument numbers, e.g. changing: function blah(input, verbose)
to: function blah(method, input, options, verbose)
only requires new default commands for method and options arguments, whereas use of nargin would need to change for input and verbose.

I find this useful -- I hope others do too! :-)