Do all seed bearing plants have flowers give me the reason why?

Coniferous plants (conifers) make seeds but they don't have flowers.

They have cones, such as pine cones, instead. The female cone produces seeds when it is fertilized by the pollen from the male cone, which is smaller and not woody.

Some coniferous plants are: pine, spruce, cedar, larch, cypress, juniper and redwood.

The reason why they evolved this way is because they found a niche for themselves where this plan worked best. What works stays; what doesn't work disappears!