Metal flashing prevents leaks where shingles meet other surfaces, like walls and chimneys. Proper flashing work takes time and know-how, so sloppy roofers sometimes slather on roof cement instead. It seals out water long enough for them to cash your check, but it soon hardens, cracks and leaks. In the end, all it does is make a proper repair more difficult. So if you see heavy 'tar' patchwork on your roof, fix it right—before it leaks and leads to interior damage.