If posts are buried in concrete foundations, you can connect them in several different ways. The plain splice simply layers the two posts together, one from either direction, and binds them using clamps or screws. But you can cut boards that will fit at right angles to wedge together, saving room. This keeps the boards strong if they are suspended in or on top of a concrete foundation.
Rebars, or reinforcing bars, are rods made from simple steel or iron components. The rods are suspended in concrete to add tensile strength -- to keep the concrete from cracking or disintegrating as easily. When rebar is laid out to be suspended in poured concrete, overlapping is proper practice. Overlapping one rod with another is common practice to avoid the weak places that would occur at each joint if the rods were just set end to end. Several inches of overlap keeps rebars functioning properly throughout the foundation.
If you are adding a concrete foundation to older construction because of a new addition, crawl space work or another project, is is advisable to reinforce old beam splices at the corners where the weight of your house is supported. Adding concrete can shift the weights and pressures of a house, so greater beam strength can be a necessary addition.
If you are applying paneling to a basement on top of a foundation wall, splices can help affix the panels at the corners to avoid gapping and warping problems. Spliced panels come with connectors that allow one panel to slide into a groove in the other panel, fastening the two together. This can be handy when installing veneers or similar layers on top of concrete.