Vladimir Putin is acting as a very strong nationalist leader in Russia. For example, he does not want to acknowledge having any real opposition; he never used to even say the name "Alexei Navalny," but instead always used vague and general terms such as "opposition" and "disruptive forces."
The highest possible election results are important for him to pose as the most popular and beloved leader in Russia without any competition. In the last presidential elections in Russia, the government made huge incentives for people to vote, mostly because of the popular opposition politician Alexei Navalny, who, after being criminally charged, was unable to be an official political candidate against Putin. He then called on his supporters to not go to vote, in order to boycott the election.
Putin does not want to look in any way weakened and create any feelings both inside and outside of Russia that he may be losing popularity at home; this is the main reason for any potential cheating from his side in the Russian election.
In the words of Vladimir Milov, main economics adviser to Navalny:
“We want to tear Putin down from his pedestal,” Vladimir Milov, an economic adviser and one of Navalny’s allies, said in an interview. “Putin will get a formal victory, but we want to make it a pyrrhic victory. We want to use the election to show that he doesn’t have as much support as he claims.”
Adding to what was commented, one of the goals in terms of the exact election outcome for Vladimir Putin was "70/70", which means to have at least 70% turnout and to gain 70% of the votes, which should ensure that he still does not have any real opposition and is not losing much popularity, as stated here:
Contrast that to prospects for Vladimir Putin, who will secure a new term in Russia's election on Sunday and whose only real concern is making sure a convincing number of voters turn out. His advisers have a goal of 70/70 - 70 percent turnout with 70 percent of votes for Putin - a threshold they believe will keep potential challengers at bay and shore up his position as Russia's only viable leader for another six years.
From The Guardian Article (article)
The opposition pointed to video evidence of voter irregularities at a number of polling stations across Russia. They included ballot stuffing and attacks on some vote observers, as well as reports of ballots being cast by “dead souls”, people who have died but remain on the electoral rolls.
In one video shared online from the Siberian region of Yakutia, voters patiently queued behind a man shoving ballots into the ballot box.
The Kremlin had pushed a broad get-out-the-vote campaign before the elections, apparently concerned that Putin’s popularity might not be enough to get voters to the polls. Incentives included raffles for prizes including iPhone Xs and cars.
From Bloomberg article(article)
The election was fake because Putin's most vocal and most politically talented rival, Alexei Navalny, wasn't allowed to run because of a trumped-up criminal conviction. It was fake because the "opposing candidates" were hand-picked by the Kremlin and because the majority of Russian media are under direct or indirect Kremlin control. It was also fake because of a fierce administrative pressure on Russia's millions of government-dependent voters -- public servants, students, workers at state-controlled enterprises -- to turn out, and because at many polling stations, especially those where the fragile Russian opposition had no observers, ballot boxes were stuffed.
There are, however, fewer reasons this time around than in several previous elections to describe the outcome as fake, too. Sergei Shpilkin, a physicist and electoral statistician who convincingly demonstrated irregularities in previous vote outcomes, noted that the vote falsification level was "likely at a record low" and close to what he'd seen back in 2004, during Putin's second, conflict-free election. According to Shpilkin, up to 8 million votes may have been added to the actual count.
I recommend reading these interesting articles that are related to your question:
Bloomberg
Reuters
The Guardian
