A surgeon’s join is useful for joining two lines of moderately unequal size — most often a tippet to a leader. It is one of the easiest joins to tie, and it lets you keep the same leader while changing tippet size to suit the fly.

Its disadvantage against a blood knot is bulk. The surgeon’s join is chunkier, and it leaves a slight angle in the line where the knot seats, so it does not run through the guides as cleanly as a slim blood knot. The blood knot is neater and slimmer, but slower to tie and fussier when the diameters do not match.

So choose by the job. For a quick join between moderately unequal sizes, the surgeon’s join is the practical knot. For a slim join in fine tippet, or one that has to pass through the guides, the blood knot earns its extra time.

What the knot does not do is cover any diameter difference at all. Moderately unequal is the limit the source gives; do not turn a convenient join into a universal one.

Source

From the Surgeon’s Join Knot, uses and disadvantages; see the knots I actually need; terms follow the FFI Fly Casting Definitions.