The Irish Seeker was about to catch it anyway.
Krum didn't decide to catch the Snitch without prompting. When Krum caught the Snitch, the Irish Seeker had seen it and was going to catch it himself. Without either cheating, or otherwise playing unfairly, there was no good way to stop Lynch from catching the Snitch other than Krum beating him to it.
“Look at Lynch!’ Harry yelled.
For the Irish Seeker had suddenly gone into a dive, and Harry was quite sure that this was no Wronski Feint; this was the real thing … ‘He’s seen the Snitch!’ Harry shouted. ‘He’s seen it! Look at him go!’
Half the crowd seemed to have realised what was happening, the Irish supporters rose in a great wave of green, screaming their Seeker on … but Krum was on his tail. ” - Harry Potter and the Goblet of Fire, Chapter 8 (The Quidditch World Cup)
Krum and Lynch crashed into each other in their battle for the Snitch. If Krum hadn't caught it, it was likely that Lynch would have, meaning that the Bulgarian team would have lost by 310 points instead of 10 points. By catching the Snitch, Krum turned it from a humiliating defeat to a more dignified defeat.
“Krum, his red robes shining with blood from his nose, was rising gently into the air, his fist held high, a glint of gold in his hand.
The scoreboard was flashing BULGARIA: ONE HUNDRED AND SIXTY, IRELAND: ONE HUNDRED AND SEVENTY across the crowd, who didn’t seem to have realised what had happened.” - Harry Potter and the Goblet of Fire, Chapter 8 (The Quidditch World Cup)
If Krum didn't catch the Snitch, his team would have lost 10 - 320 instead of 160 - 170. By beating Lynch to it, he made it a much lesser defeat.
Ginny Weasley did the same thing during a Quidditch match at Hogwarts where she was the substitute Seeker for Harry, and the Gryffindor House Quidditch team without Harry wasn't actually very good.
“It was hard to say what the worst thing was: Harry thought it was a close-run contest between Ron’s fourteenth failed save, Sloper missing the Bludger but hitting Angelina in the mouth with his bat, and Kirke shrieking and falling backwards off his broom when Zacharias Smith zoomed at him carrying the Quaffle. The miracle was that Gryffindor only lost by ten points: Ginny managed to snatch the Snitch from right under Hufflepuff Seeker Summerby’s nose, so that the final score was two hundred and forty versus two hundred and thirty.” - Harry Potter and the Order of the Phoenix, Chapter 26 (Seen and Unforeseen)