Horizontal top tubes are more comfortable to lean against if you are feeling lazy.
They are easier to shoulder (cyclo-cross style) and carry the bike.
On smaller sizes, a horizontal top tube places the seat cluster and the rack mounts higher than for sloping tubes. This means that rack stays can be in-line with the rack top, maintaining triangulation and stiffness.
Horizontal TT provides clearance so your dynamo front lamp (on the fork crown) does not intersect with the top tube if your bars flip. I have cracked expensive lamps on my sloping TT.