PLA is an excellent filament for 3D printing because the glass transition temperature - where it becomes amorphous and moldable - is relatively low at 60-65°C. Additionally, because PLA is made out of starch and sugar cane by-products, the smell of hot PLA is pleasantly sweet, unlike hot ABS. Again, because it's made out of by-products, PLA is one of the cheapest filaments readily available for home 3D printing. Sugar cane and other starch products are, of course, renewable resources, so PLA is a renewable product. It is also biodegradable.