the most active type of LP in emerging VC funds is the GP of another fund.
not by committed $, but by count. the number of GPs allocating to small funds right now (especially in ecosystems like SF) is insane.
large firms invest through dedicated fund-of-funds vehicles/programs inside the firm (e.g. bain, GC, felicis), through the family offices of their senior partners, or through GPs directly with a few goals: stay close to what's trending, get access to information, and build a diversified early-stage pipeline (that large firms simply can't cover efficiently).
even small/mid-sized firms use this as part of their strategy, sometimes right in the pitch ("we'd rather write a few small LP checks into sub-$20M funds than hire a team of associates").
yes, the value of these investments is high: they sit outside a standard LP allocation workflow, they act as a catalyst for first close, and they often send a strong signal for the entire fundraise ("Marc / Josh are LPs in my fund"), but they come with caveats like re-up risk, closed-loop capital circulation, priority deal access, etc.
imho, it's closer to a call option on deal flow than a classic LP position, but if you're raising your first fund, it makes sense to go to your VC peers before pitching the rest of the world.