Why are these animes called so? (something;something) There are manga/anime called "something;something", what for? does it server any purpose for identifying it as a series? example: steins;gate, robotic;notes.
The Wikipedia entry for Steins;Gate begins with the following two sentences (emphasis mine):


  Steins;Gate (シュタインズ・ゲート Shutainzu Gēto?) is a Japanese visual novel developed by 5pb. and Nitroplus and released for the Xbox 360 on October 15, 2009. It is the second game in 5pb. and Nitroplus' Science Adventure series following Chaos;Head and was succeeded by Robotics;Notes.




The entry for Robotics;Notes is fairly similar:


  Robotics;Notes (ロボティクス・ノーツ Robotikusu Nōtsu?) is a Japanese visual novel developed and published by 5pb.. The game was released on the PlayStation 3 and Xbox 360 video game consoles on June 28, 2012, with a PlayStation Vita port released on June 26, 2014 under the title Robotics;Notes Elite. It is the third game in 5pb.'s Science Adventure series following Chaos;Head and Steins;Gate and is described as an "Augmented Science Adventure" (拡張科学アドベンチャー Kakuchō Kagaku Adobenchā?).




Finally, the entry for Science Adventure reveals the following:


  The Science Adventure series (科学アドベンチャーシリーズ Kagaku Adobenchā Shirīzu?) is a Japanese video game franchise created by 5pb. in collaboration with Nitroplus. The series currently consists of four main visual novel games; Chaos;Head, Steins;Gate, Robotics;Notes, and Chaos;Child which have each received various ports, sequels, and spin-offs, as well as various manga and anime adaptations.




So, yes, it is as you assumed: They're part of the same series, made by the same company.

As for why the developers decided to use a semicolon in the name: why not?