All over you see the use of cheap tar like sealants - used a lot on roofs where a newer flat porch roof joins the older existing roof. I see this a lot in chicago land. The problem with sealants is clear in the picture- THEY ALWAYS FAIL IN TIME - they shrink and crack and seldom does a home owner re apply it. So When the stuff gets weathered, it will allow rain/melting snow to enter where it should not- then you could have wood rot and moisture damage which could lead to growth of substances.