Every organization needs to clearly understand its internal strengths and weaknesses, as well as its opportunities and threats. Strengths and weaknesses look are internally-focused assessments, while opportunities and threats look externally to focus on what's happening in the market.
A deep understanding of these four elements creates an effective foundation for strategic planning. While quantitative data is the backbone of strategic planning, metrics alone aren't enough to build an effective plan. The qualitative data the emerges from a SWOT analysis provides the context for the execution of your strategic plan - the landscape you're executing in, and the internal strengths and weaknesses you have to help you or navigate around.
These insights are critical to effective strategy execution. If you don't account for competitive threats, for examples, your plan may be sidelined when a competitor introduces a new product that makes yours obsolete. It'll be impossible to execute on the initiatives you set out, because you'll be busy building a whole new plan!