I thought linear algebra was pretty much the poster child of applied mathematics - the entire field was invented to represent computations in a regularized form to feed into computers. Well not really, but much like Boolean algebra or the Fourier Transform, it was pretty much a curiosity until computers came along.
Many important advances in linear algebra happened in WWI to solve optimization problems for logistical and industrial planning. A lot of these applications boil down to high-dimensional systems of linear equations, which back then were solved by hand. Efficient algorithms translate to reduced labor costs.
