Depending on the beam’s weight, where it impacts, and what kind of tank it is, here are some possible effects:

1. Absolutely nothing
2. Cause the crew to button up, reducing their visibility
3. Draw the crew out to assess or repair damage, expsoing them to small arms fire
4. Remind the crew that their enemy is active in the area and slow the advance
5. Distract the crew long enough for infantry to employ a more effective weapon, possibly including approach the tank on foot without being seen
6. Main gun barrel impact - deform the barrel enough to disable it
7. Turret impact - disable turret rotation, reducing aiming capability
8. Track impact - mobility kill by damaging roadwheels and/or track, or just getting stuck there
9. Direct side impact - probably nothing
10. Random gear impact - even old tanks have external mounted machine guns, optics, radio antennae, and shit that could be disabled
11. Exposed crew impact - squishy squishy

Of all these possibilities, I think just trying to get the damn thing stuck in the wheels/tracks is the most likely to actually work, even on relatively modern tanks.