TIME SAVING
Being a web designer, my main reasoning is simply the time it saves not having to create
different sites for each different device. When mobile and tablets first started becoming
the "next best thing since sliced bread", every client wanted us to update their site and
include a mobile and tablet version... and, of course, "it shouldn't cost any more because
it's all part of the design, right?" BS...
IMPROVED FUNCTIONALITY
Ever since you could start viewing sites on your cellphone, even the cheapest flip phones
initially, I started designing to appeal to both computer and mobile users, JUST IN CASE, but,
I was then also very limited as to what type of functionality I could include, but that often
meant a lack of features for computer based visitors, lol.
But more importantly, MY CUSTOMERS BENEFIT THE MOST
The benefits for my customer actually outweighs the benefits for me. Designing "responsive"
sites not only ensures cross-device accessibility (without sacrificing anything) but it also helps
with sales and conversions. Plus, it saves them money in terms of SEO. Prior to responsive sites,
you'd have to, essentially, have 2 sites, which meant more time spent on SEO and more money
spent as well.
Nope, responsive sites make my primary job so much easier, has many benefits for my customer
and yeah, ok, there's also that little "Google Recommends" factor as well.