By mixing the information you have and outlining the concerns from PEST and SWOT analysis, you can grasp both outside influences (on a macro-level) and internal factors (on a micro-level). Even though you can’t influence PEST factors, you can take advantage of it with SWOT analysis.
If you have a business, you should be using SWOT analysis. It’s really that simple. If you decide to use SWOT analysis only a couple of times for your business, be sure to do it these two ways.