As the owner of copyright of your work, nobody may use your work without permission. In recent years, publishers and competition organisers very frequently embed clauses within terms and conditions that take unjustifiably extensive rights in any work that you submit to them. It is common for such clauses to assert blanket permission for any commercial use including the ability to sell on to other publishers or exploit in other media, and to waive your moral rights to a byline or to object to misuse.
Despite such T&C nearly always asserting that 'you retain copyright in your work' effectively the rights taken are as extensive as copyright itself, They just steal the economic value. Always read T&C carefully so you know exactly what you are agreeing. Also be on the alert for liability clauses that make you legally and financially responsible for actions of the publisher that are beyond your control.
The reason for rights grabs is of course that the 'grabber' wants the photograph to be their financial asset, not yours. Amateurs who don't think of their work as having monetary value should consider that it certainly does have value to the 'grabber', and they are effectively demanding a blank cheque. This is generosity they would be unlikely to bestow on a needy stranger in the street, let alone well-heeled corporates and their shareholders. Professionals should recognise that their work is the only asset they will ever own, and giving it away c/o 'rights grabs' is a fastrack to insolvency in the same way that 'Buy One Get Twenty Free' would be a dumb way to run a shop.