Why all (or almost all) devices are has special transceivers such as this?
Why aren't they embedded into Fibre Channel card, switches and other devices?
Basically it means you can buy a FC card or a switch, and the SFP to match your fiber needs, rather than needing multiple variants of FC cards/switches for each fiber type/distance/wavelength - much less the combinations that are possible.
As a basic example, you might have a switch that has an 80Km single-mode SFP connected to a datacenter in another city, a 10km singlemode SFP connected to another building on the same campus, and a 1Km multimode SFP connected to another switch in the same building.
In fiberchannel, you might have a bank of disks connected by short-range multimode in the building, another connected by short-range singlemode in the area, and a third connected by long-range singlemode in a data bunker somewhere far away.
Without even getting exotic, you have 850 and 1300 nm multimode, 1310, 1490 and 1550 singlemode. 1620 singlmode is a bit more exotic, but not uncommon. You have various ranges (affected by both power out and reciever sensitivity.) You have bi-directional versions, transmitting both directions over one fiber at different wavelengths. Then you can diverge into exotic options....