Piano transcriptions help promote the popularity of large orchestral works. In the 19th century, it was hard for newly created symphonic works to reach ordinary people, because majority of the population had few chances to hear them in concert halls. For one reason, it was hard to travel to large cities because of poor transportation; it also takes a lot of preparation to stage such a performance; therefore performances of symphonic works were rare and for that matter they were usually regarded as important cultural events. Things got better with the advent and popularization of pianos, because now families across Germany and France were able to play and enjoy these works in their homes. It only takes a piano and a score, and you can hear symphonies by mozart, beethoven, and haydn. That's basically the reason Liszt made these piano transcriptions. It is rather out of necessity that he did this.