The checking could be for a number of reasons, from wood type to cut to moisture or installation issues. Without seeing it in person or seeing pictures (with a scale), it's tough to say for sure how big of an issue it could or could not be. That said, 1/4" width is starting to get a bit wide for the sake of moisture intrusion (insects and mold too).

The big concern is the depth of the cracks. When a beam is chosen for that use, an engineer will calculate the load it needs to carry and assign a cross section given the type of wood and length of post. When the crack starts getting deeper, you start to lose cross-section, thus strength (you also start getting into issues for bending and twisting, but for this application, we probably don't need to go that deep). It depends on cost considerations as well as who actually "engineered" the thing at the time of building, but they typically use a healthy safety factor.

In the short term, you can get a good outdoor wood sealant and coat the beam as needed (get it into the cracks the best you can, even as they expand). Keep an eye on the depth of the checks as well as the outside dimensions of the beam (you said it's an 8" post so it should measure somewhere near 7.5" per side); you don't want it to bulge here.

OA

Source: Civil Engineering degree - I do highways now - really a bit of everything, but studied to design bridges. Wood is a bit different from steel and concrete however, so I would defer to someone who knows more in that area.