"Get offers from other companies" mean you are doing a job search and interviewing to get a job in that company. Once they offer you a job, you get that offer letter. Most people do this when they are already fed up with their current employer and are ready to move on. So this advice means -- actively search (best time to get a job is when you have a job).
Pitfalls with this advice: your current employer is not dubious. They may know you are actively searching. In addition, once you have an offer and you bring this up -- your current employer will "try" to match that offer to try to keep you. Why? Often times this is the default to avoid disruption and often times you are well-liked and they actually do want to keep you. They may also try to "keep you temporarily" -- until they can replace you. Once you have said "you have an offer" it acts as a flag to you. Which can also mean you may be the first to go if anything happens such as layoffs. Thread carefully.
You should interview defensively at least a couple times a year IMO. It is good to know your market value. HR knows your market value. When your pay and your market value go wildly out of line, it's time to step up. This happens disturbingly often with long-term risk-averse employees.
That said, if you can't stomach risk, then don't do this. But try to understand that by not taking risks now, you're taking a huge long-term risk on your financial future.
"Once you have said "you have an offer" it acts as a flag that at you." I've found this line of thinking to be false but so many employees are afraid of this. If it does happen you just leave again, big deal.
Well, your manager and your boss's manager may KNOW that you had a job offer and is "wanting to leave the company". In addition HR flags you as well, perhaps they make notes about it, especially when they have to counter the offer. This is not really false thinking, this is how it works! Whether they blatantly flag you or they have it in their head, it acts as a "flag". If you are very well-liked and indispensable, it may be a good thing, since they will do what they can to make you happy moving forward. If you are not on the good side, you have to now understand that your manager and your manager's manager are informed that you had the intention of leaving.
Pitfalls with this advice: your current employer is not dubious. They may know you are actively searching. In addition, once you have an offer and you bring this up -- your current employer will "try" to match that offer to try to keep you. Why? Often times this is the default to avoid disruption and often times you are well-liked and they actually do want to keep you. They may also try to "keep you temporarily" -- until they can replace you. Once you have said "you have an offer" it acts as a flag to you. Which can also mean you may be the first to go if anything happens such as layoffs. Thread carefully.