A phone showing full bars can fail to load a page while one showing two bars works fine. The indicator measures something real and largely unrelated to usable performance.
Bars measure strength, not quality
The indicator reflects received signal power from the nearest tower. It says how loud the signal is, and nothing about how clearly it can be understood.
A strong signal accompanied by strong interference carries less usable data than a weaker signal in quiet conditions, in the same way a loud conversation in a noisy room is harder to follow.
Networks measure this separately as signal quality, and phones generally do not display it despite it predicting performance better than strength does.
The mapping to bars is not standardised
Manufacturers choose which signal levels correspond to which number of bars, so identical conditions produce different displays on different handsets.
Thresholds are usually set generously, meaning bars remain high across a wide range of conditions and drop only when the signal is genuinely poor.
This is why comparing bars between two phones side by side reveals more about their display logic than about the network they share.
Congestion is invisible to the indicator
Capacity at a tower is shared among everyone connected to it. When many devices are active, each receives less throughput regardless of individual signal strength.
A crowded venue is the clearest case: signal is excellent because the tower is close, and nothing works because the capacity is fully committed.
Nothing in the display changes to reflect this, since the phone's received power is exactly as it was when the area was empty.
The backhaul can be the limit
Every tower connects onward to the core network, and that link has finite capacity. If it is saturated, improving the radio connection changes nothing.
This is a common constraint at sites upgraded for higher radio speeds without a corresponding upgrade to the connection behind them.
What would be more informative
Signal quality measurements, the frequency band in use and the current network technology all predict experience better than strength alone.
Most phones expose these in diagnostic screens, which is where the numbers behind the bars actually live.
Reading them makes the common situations legible: high strength with poor quality indicates interference, while good strength and good quality with slow speeds indicates congestion somewhere further along.