According to the film's official novelisation, Ben Kenobi recognises that the mission has a non-trivial amount of risk involved. Having twice as many participants (especially a young eager pilot with excellent marksmanship, superior physical prowess and who's strong in the Force) more than doubles their chances of success.
Note the additional dialogue.
“I need your help, Luke,” Kenobi explained, his manner a combination
of sadness and steel. “I’m getting too old for this kind of thing.
Can’t trust myself to finish it properly on my own. This mission is
far too important.” He nodded toward Artoo Detoo. “You heard and saw
the message.”
Star Wars: A New Hope - Official Novelisation
Had Luke not come with him, Obi-Wan would have died on the Death Star and although Vader would have probably still engineered Leia's escape on the Falcon, without Luke to fly his X-Wing, the Rebellion's attempts to destroy the Death Star would have been in vain.
Ben also thinks that he can see the will of the Force in Luke bringing the droids to him.
The old man suppressed a smile, aware that Luke’s destiny had already
been determined for him. It had been ordained five minutes before he
had learned about the manner of his father’s death. It had been
ordered before that when he had heard the complete message. It had
been fixed in the nature of things when he had first viewed the
pleading portrait of the beautiful Senator Organa awkwardly projected
by the little ’droid. Kenobi shrugged inwardly. Likely it had been
finalized even before the boy was born. Not that Ben believed in
predestination, but he did believe in heredity—and in the force.
Star Wars: A New Hope - Official Novelisation
