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Pressure has more competing everyday units than almost any other physical quantity, and which one you encounter depends entirely on the industry, not the physics. The pascal (Pa) is the SI-standard unit — exactly one newton of force per square metre — but a single pascal is such a tiny amount of pressure that it's rarely used alone; engineering and meteorology work in kilopascals (kPa) or hectopascals (hPa) instead. PSI (pounds per square inch) dominates in the US and Canada for tire pressure, hydraulics and industrial equipment, while most of Europe, Australia and Asia use kPa for the same purpose — so a tire pressure sticker showing "32 PSI" on a US car becomes roughly "220 kPa" on the equivalent spec sheet elsewhere, and mixing the two up when checking or setting tire pressure is a genuinely common and consequential mistake.
Bar and atmosphere (atm) both roughly approximate sea-level air pressure, which is why they're the intuitive choice for contexts tied to "normal" everyday conditions: 1 atm = 101,325 Pa exactly, defined as a fixed reference value, while 1 bar = 100,000 Pa is a clean round metric number close to (but not identical to) 1 atm — European weather reports commonly use millibars (equivalent to hPa) for exactly this reason. Medicine sits entirely outside this metric-vs-imperial split: blood pressure is measured in millimetres of mercury (mmHg), a unit dating back to early mercury-column barometers, because clinical blood pressure standards were established using that instrument long before pascals existed and the convention has never been replaced, even though mmHg has no direct SI equivalent without a conversion factor.
It's a regional convention split, not a technical difference — North America predominantly uses PSI (pounds per square inch) for tire pressure and hydraulics, while Europe, Australia and most of Asia use kPa. The same physical tire pressure of roughly 32 PSI converts to about 220 kPa, and mixing up the two units when checking or inflating a tire is a common, consequential mistake.
1 atmosphere is defined as exactly 101,325 pascals — a fixed reference originally tied to average sea-level air pressure. 1 bar is defined as exactly 100,000 pascals, a clean round metric number chosen for convenience that happens to be very close to, but not identical to, 1 atm. The roughly 1.3% difference rarely matters for everyday use but does matter in precise scientific and engineering contexts.
Because clinical blood pressure measurement standards were established using mercury-column sphygmomanometers long before pascals became the standard SI pressure unit, and the mmHg convention has simply persisted in medicine ever since. It has no simple round-number SI equivalent — 1 mmHg equals approximately 133.3 pascals — which is exactly why it needs its own dedicated conversion rather than a clean metric relationship.
Because one pascal is an extremely small amount of pressure — roughly the pressure of a dollar bill resting flat on a table. Practical engineering and meteorology contexts almost always use kilopascals (kPa, thousands of pascals) or hectopascals (hPa, hundreds of pascals, common in weather reporting) instead of the bare pascal unit.