1. L1 visas are intra-company transfers that a company exists in US and another foreign country can issue a L1 visa to their foreign employees to enable them work in US. L1 visa holders are not allowed to switch company while they are in US. If they do so, the visa sponsor may cancel their visa and they should leave US. H1B is also quite similar to L1 but you can switch company on H1B.
L1 is more beneficial than H1 because a dependent of a L1 visa holder can work in US with a EAD where a Dependent visa holder of a H1B visa holder is not allowed to work.
2. Since L1 has a restriction that you should stick to your company, people prefer H1B most.
3. I believe so. But it always depends on your company policies whether to offer a GC for a L1 employ or not.

