Seniority at an airline mostly determines the quality of life (QOL) that a pilot will have. When you first start you are near the bottom of the seniority list. In "good" times (when airlines are hiring) the distance from the bottom grows and life gets better. The only way to move up the list is for someone above to die, quit, or get fired.
In addition to total airline seniority there is seniority by status as well. For the most part a new pilot at an airline starts off as a First Officer (there are a few rare cases where pilots start off as a Captain...called as "street Captain"). Take an airline with 1000 pilots. Of that 1000 there are likely 550 Captains and 450 First Officers. There are typically more Captains than First Officers in a given status as the Captains are pulled away for training twice as often as First Officers. For my example Captains are seniority numbers 1-550 and First Officers are numbers 551-1000. The First Officer with seniority number 551 is living a pretty good life. He is the most senior First Officer and can have his first pick of trips/schedules each month. If there is ever a month he wants to sit at home on reserve and not fly he has a really good chance of doing so (all the while getting full pay!). In addition to all of that he is just one number away from making Captain. One would assume he would jump at the first Captain opening he could. You know what assuming does right?
If the First Officer with seniority number 551 moves up one number to 550 and bids a Captain seat, he will become the most junior Captain. The good life of getting weekends off, his pick of the lines, and predictable day to day schedule are gone. He will have a similar QOL as the First Officer just hired to fill his vacant First Officer seat.
Over time the new Captain will move up the seniority list again and one day might hold the same QOL as he held as a First Officer. When times are good people are moving up the list quickly. When times are bad..they can become stagnant. Because of this some pilots hold off bidding Captain right away until they can have a better QOL. This can be risky.
If the same First Officer who held seniority number 551 decided to stay a First Officer, and the guy right under him number 552 bid Captain, number 552 would get it. I've been told many stories of such scenarios happening. Number 552 will get Captain pay and that coveted Turbine Pilot in Command time while number 551 hops along. If number 552 is the last Captain to upgrade for a few years it could drastically change his and number 551's lives.
The 552 is Captain for 3 years while times are bad. Once times get good and airlines are hiring like gangbusters, number 552 could get a better paying job at a bigger airline as he has 1000's of flight hours of Turbine PIC. Meanwhile number 551 sucks it up and bids Captain and is now 3 years behind someone that was junior to him.
This is a very simplified example of seniority and QOL. Thing's can get complicated quickly with bigger airlines which have several plane types and domiciles. Number 551 might live and be based in San Francisco. He might be able to hold Captain only in New York. Number 552 might live in New York. Is it worth the cross country commute to hold Captain? Some think it is....some will wait until they can hold Captain where they live. To some QOL is more important than pay.
Relating this to my current situation. I am the most junior in my status and thus I get airport standby for my assignments. If I were to transfer to another base I would be senior enough to hold a hard line. Doing so would mean a more predictable schedule and possibly better pay. I am not transferring as I would have to commute to the new base and would have less time at home. For me QOL is more important than pay. I may sit airport standby everyday and rarely fly, but I get to drive home each night to my wife, house and bed. If I commuted I would likely have much less time at home. More about commuting in an upcoming blog.