As an extension of the formal version of the auxiliary verb だ, である, でもある simply conveys that something 'is also (B)'. However, as も often shows something that is surprising or unexpected, でもある will primarily be used to describe additional facts about (A) that may be unexpected. Due to this, でもある usually fills the (B) role in sentences like 'while (A), it is also (B)', or 'in spite of (A), it is also (B)'.
でもある may be used after nouns, or な-Adjectives.

• タナカさんは英語（えいご）の先生（せんせい）だし、休（やす）みの日（ひ）は船（ふね）の船長（せんちょう）でもあるから休（やす）む時間（じかん）がない。
  While Tanaka-san is an English teacher, he is also the captain of a ship on his days off.
• これは洗濯機（せんたっき）でもあるけど、同時（どうじ）に乾燥機（かんそうき）でもある。
  In spite of this being a washing machine, it is also a dryer.
• バンジージャンプは楽（たの）しいけれども、同時（どうじ）には危険（きけん）でもある。
  In spite of bungee jumping being fun, it is also dangerous at times.
• この機械（きかい）は重（おも）くて邪魔（じゃま）だけど、同時（どうじ）に便利（べんり）でもある。
  While this machine is heavy and a nuisance, it is also useful.