I recognize that this question asks for speculation about the internal state of mind of Bashar. While there are many possible motivations, you ask for "real political" reasons that are "realistic". I don't think you care about what I think Barshar could have been thinking, so I will defer to the opinion of experts, and list some possible motivations that have been mentioned in other reliable sources.
- It could just be a "crazy move".1
- It could be part of a strategy of escalation against civilians.2
- It could be that Bashar is war-weary and wants to win quickly. Their resources are depleted and their army exhausted. And, he thought that "the risk of retaliation for a major chemical weapons attack was falling".3
- "It could be that the attack in Idlib was the work of a rogue or a madman."4
- It is possible that Bashar didn't fear any retaliation, and the attack could have been calculated to help win the war more quickly, staying in power through the reconstruction.5
- Bashar could simply be asserting his strength.6
1. Anne Barnard. The Grim Logic Behind Syria's Chemical Weapons. The New York Times. "One of the main defenses offered by Mr. Assad’s allies and supporters, in disputing that his forces carried out the strike on Tuesday, is that such an attack would be “a crazy move,” as one Iranian analyst, Mosib Na’imi, told the Russian state-run news site Sputnik."
2. Id. "it is part of a carefully calculated strategy of escalating attacks against civilians."
3. Emma Graham-Harrison. Syria nerve agent attack: why it made sense to Assad. The Guardian.
4. Thanassis Cambanis. What Could Possibly Motivate a Chemical-Weapons Attack? The Atlantic.
5. Id. "If he can drop chemical weapons on the same day that a conference in Brussels is discussing plans to reconstruct Syria, without any substantive response, then he’ll inch even closer to his current goal of winning a Western-funded rebuilding plan on his own terms."
6. 'Chemical attack' in Syria draws international outrage. Al Jazeera. "They [chemical weapons] are not really serving a substantial military purpose. They can certainly spread terror, and they certainly seem to have a political effect, although it's hard for me to calculate how that would be a positive thing for Bashar al-Assad right now - unless he wants to demonstrate that he's there, no matter what anyone else says or wants to do about it."