For many properties, boron has an intermediate behavior between carbon and silicon. For a few properties, boron resembles more phosphorus than silicon or carbon (mainly because of a closer ionic size, which makes borates somewhat intermediate between phosphates and silicates).

But both carbon and silicon are extremely cheap and abundant, many orders of magnitude more abundant than boron. Even phosphorus is several orders of magnitude more abundant than boron.

So in many cases there are carbon and/or silicon compounds (or sometimes phosphorus compounds) with properties not very different from some boron compounds. For instance in some applications where boron nitride or boron carbide would be desirable one of diamond, graphite, silicon nitride or silicon carbide may also be acceptable.

Therefore the boron compounds are typically used only when their specific benefits are so great that they overcome any cost difference over possible carbon-based or silicon-based or phosphorus-based substitutes.

In living beings (e.g. in plants), the role of boron is similar to that of phosphorus, both are used in their oxidized form, i.e. as phosphates or borates, which both have an affinity for binding to carbohydrates (like phosphate in nucleic acids) or sometimes to other alcohols (like in cellular membranes).