I guess the expectation is that students will undertake work experience though vacation schemes and mini-pupillages, which are timed to occur in the vacation periods. Also as you wouldn't yet have a law degree there would likely be some limitations on what you could practically do; it may be law firms would find it hard to justify paying a student for a full year if they have a very restricted range of things they could do.
Also the training contract/pupillage is the year industry of a sorts I suppose, since you won't be a fully qualified lawyer or barrister until after completing that. Additionally there are (or maybe were) restrictions on how long you could take to complete a law degree after beginning one and have it count as a qualifying law degree (QLD), which was I think seven years. So adding an extra year in the mix could make things a little narrower to complete within that timeframe if there are serious (non-academic, extenuating) issues that arise which delay things.