Sites like Glassdoor are useful, but what they don't provide is company-specific detail. I can view Amazon's average SDE salary, but is that figure for a job level 4 or job level 5? (At Amazon, pay is based on job levels. Two people can hold the same position but have different job levels. For example, Area Managers who are promoted from hourly positions start at level 4, but new hire AMs come in at L5).
Other things Glassdoor can't provide:
-Pay increase percentages for internal promotions
-Pay increases and stock award amounts for annual performance reviews based on performance grade
-Compensation data broken down by race, national origin, native language, etc.
-Differences in pay for positions based on team, department, geographic location, etc.