Reducing cost is perhaps the most often cited reason for hiring sub-consultants to perform tasks that otherwise would be performed in house. Sub-consultants don't get paid benefits, don't take sick days, or vacation, and when the project is completed, so is the outflow of cash. But there are other good reasons to outsource tasks to others.
1. Efficiency - hiring someone who specializes in one thing all of the time can lead to shorter timelines for the task or project. There is no learning curve, mistakes that someone still learning would make are avoided, and you will often get better results.
2. Perspective - many companies suffer from institutionalized thinking and ways of doing things. They perform tasks or projects the same way each time because that is the way they have always done it or it is the only way they know how to do it. Sub-consultants can bring in fresh ideas, and can offer advice that is born from working in many different companies and seeing how they do things. Having a new perspective can excite your employees and motivate them to wonder what else they should be learning.
3. Risk - Because a specialist often has a broad range or resources and experience to pull from, the risk to your project is reduced. Spreading the risk to the sub-consultant also frees up your own resources to be less timid about making mistakes which are essential to learning. As an example, a third-party partner often has one specialty and because of that, is more likely to be up to speed on changes in the industry and regulations. Knowing the latest, and taking it into account, reduces the risk to the company.
4. Scaleability - The ability for your company to expand or contract is often a key to fiscal soundness. Consultants do not expect to be retained forever and letting them go is far easier than letting employees go when business slows. In essence, your are sharing the consultant (and their salary) with other companies. Consultants can be easily scaled back to only be available as the work dictates, whereas employees are getting paid regardless of the workload.
5. Focus - Let's face it, employees do not often have the luxury of being removed from their full time responsibilities to focus on one project or task. Even if that is tried, the reality is that they often get pulled back into their "real job" when things get tough. This waters down the ability to focus on the task at hand and thus the project suffers. Having a sub-consultant on the project provides someone who is totally focused on what is important. Things are much less likely to fall through the cracks when it is someone's full time job to make sure they don't.
6. Morale - tied into several of the reasons above is employee morale. Morale can suffer when employees are overworked. It can really take a hit if the business slows down and words like "layoff", "rightsizing", or "fired" start getting thrown into the daily conversation. Employees that know the company is thinking about fiscal responsibility will have better feelings about how the company is being managed. Hiring sub-consultants is being fiscally responsible.