Your birth chart shows what is in your chart. Dashas show what is active right now.
Two people born in the same year can have similar natal placements but be in completely different dasha periods — meaning completely different seasons of their life, even at the same age. A generic transit-only forecast cannot tell you that. Dasha timing can.
Vimshottari dasha is a 120-year cycle made up of nine planetary periods. You enter that cycle at a specific point at birth based on your Moon's nakshatra and its exact position within that nakshatra.
Each major period — called a mahadasha — subdivides into sub-periods called antardashas, which subdivide further. The result is a layered timing system that can be used to understand not just which year something is likely, but which months, and sometimes which weeks.
Most mainstream astrology apps are built primarily around Western natal astrology and transits. Western astrology has its own predictive techniques, but dasha timing comes from Jyotish and is rarely part of the mainstream app experience.
The result is that a huge number of people who already use astrology have never been introduced to the timing architecture available in Jyotish.
Different, not better. They do different jobs.
Transits describe what the sky is doing for everyone. Dashas show what is specifically active in your chart right now. The combination is where real timing comes from — the dasha shows the season, transits help show when the active material becomes acute, visible, or event-level.
Both matter. For event timing, I do not treat either one as the entire answer.