For exterior trim work and siding, pre-engineered lumber has several key features that make it a practical option. It's pre-primed so that it cuts down on paint time. It's treated with EPA-approved preservatives, giving it extra resistance to decay and insect damage. There are no knotholes or imperfections, and it's easy to cut and drill.
For more on siding, consider:
Guide to Exterior Siding
Siding Roundup: 8 Options to Beautify Your Home
Everything You Need to Know About Engineered Wood Siding