I've always used spirit levels and very rarely my iPhone to level things, but with both you don't have a way to 'set' the spirit level. This allows you to set the zero plane anywhere you like, making finding rafter angles 10 times easier than digging up a combination angle and measuring.
π 60MM High Precision Horizontal Bubble Cardan Level for Accurate Measurements
9 Review
π΅ IRWIN Tools Bullseye Level (1794487) - Blue
9 Review
π SKIL 24-Inch Digital Level - LV941901
8 Review
Johnson Level & Tool 40-0912 Self-Leveling Cross-Line Laser Level, Red, 6 x 7 Inches
10 Review