This "impeachment investigation" is a political act to help their candidate win the next presidential election.
Democrats themselves describe it in this way. For example, here is how Rob Kall, the director of one of the progressive Democrat websites, described the purpose of the investigation:
"The idea should be to keep the impeachment going as long as possible, with new testimonies and new releases of disclosures of alleged corruption and treason on a regular basis.
"Looking at impeachment as a process for removing the president is the wrong way of thinking about it. Looking at it as a key that gives access to investigative tools is the smarter, more strategic, way of looking at it.